Library 


Engineering  Science  Series 


THE   HYDRAULIC   PRINCIPLES   GOVERNING 
RIVER  AND   HARBOR   CONSTRUCTION 


ENGINEERING  SCIENCE  SERIES 

EDITED  BY 

DUGALD  C.  JACKSON,  C.E. 

PROFESSOR  OF  ELECTRICAL  ENGINEERING 

MASSACHUSETTS  INSTITUTE  OF  TECHNOLOGY 

FELLOW  AND  PAST  PRESIDENT  A.I.E.E. 

EARLE  R.  HEDRICK,  Ph.D. 

PROCESSOR  or  MATHEMATICS,  UNIVERSITY  OF  MISSOUBI 
MEMBER  A.S.M.E. 


THE  HYDRAULIC  PRINCIPLES 

GOVERNING 

RIVER  AND  HARBOR 
CONSTRUCTION.. 


BY 
CURTIS   McD.   TOWNSEND 

COLONEL,    UNITED   STATES   ARMY    (RETIRED),    MEMBER,    AMERICAN 

SOCIETY  OF  CIVIL  ENGINEERS,  WESTERN  SOCIETY  OF  ENGINEERS, 

ST.    LOUIS   SOCIETY   OF   ENGINEERS,    ETC.;     LATE   PRESIDENT, 

MISSISSIPPI   RIVER   COMMISSION 


gorfe 

THE  MACMILLAN  COMPANY 
1922 

AU  rights  reserved 


FEINTED  IN  THE  UNITED  STATES  OF  AMEKICA 


Engineering 
Library 


COPYRIGHT,  1922, 
BY  THE  MACMILLAN  COMPANY 


Set  up  and  electrotyped.        Published  July,  1922, 


Press  of  J.  J.  Little  &  Ives  Co. 
New  York 


CONTENTS 

CHAPTER  I 

PAGES 

INTRODUCTION 1-4 

CHAPTER  II 

THE  FORMATION  OF  RIVERS     ........  5-14 

The  Ocean,  the  Origin  of  the  Water  Supply  of  Rivers  —  The  Sur- 
face Flow  —  The  Movement  of  Eroded  Material  —  The  Subterra- 
nean Flow — The  Formation  of  an  Alluvial  Valley — The  Influence  of 
Snow  on  the  River's  Discharge  —  The  Area  of  Deposition  —  Glacial 
Action  —  The  Formation  of  the  River  Channel . 

CHAPTER  III 

LAWS  GOVERNING  THE  FLOW  OF  WATER  IN  RIVERS        .         .         .   15-23 

The  Derivation  of  Hydraulic  Formulas  —  The  Coefficient  of 
Roughness  —  Minimum  and  Maximum  Critical  Stages  —  Diffi- 
culties of  Applying  Hydraulic  Formulas  to  River  Channels  — 
Modifications  Due  to  the  Velocity  of  Approach  —  The  Flow  of 
Water  in  Bends  —  The  Effect  of  Obstacles  on  the  Flow  of  Rivers  — 
The  Flow  Near  the  Mouths  of  Rivers. 

CHAPTER  IV 

THE  FLOW  OF  SEDIMENT  IN  NON-TIDAL  RIVERS     ....  24-34 

Material  in  Solution  —  Material  in  Suspension  —  Relation  of 
Slope  to  Degree  of  Saturation  —  Movement  of  Material  Along  the 
River's  Bed  —  Sand  Waves  —  Proportion  of  Material  in  Suspen- 
sion and  Moving  as  Sand  Waves  —  Movement  of  Material  in  Bends 
and  in  Straight  Reaches  —  Resultant  River  Sections  —  The  Radii  of 
Curvature  of  Bends  —  Deposition  of  Sediment  Caused  by  Dikes  — 
Depth  in  Pools  a  Function  of  a  River's  Discharge;  That  on  Bars  of  its 
Slope  —  A  Comparison  of  the  Flow  of  Sediment  in  the  Strait  Con- 
necting Lake  Huron  and  Lake  Erie  with  that  in  the  Mississippi 
River  —  The  Effect  on  Channel  Depths  of  the  Inclination  of  the 
Axis  of  the  Bar  to  that  of  the  Channel  —  The  Flow  of  a  River  Sum- 
marized. 

CHAPTER  V 

A  RIVER'S  DISCHARGE  AND  FLOOD  PREDICTION       .  .         .  35-43 

The   Discharge  Curve   of   Conduits   and  its   Equation  —  The 

813606 


VI  CONTENTS 

PAGES 

Mean  Discharge  Curve  of  a  River  —  The  Danger  in  its  Application 
—  The  Influence  of  a  Tributary  on  the  Discharge  Curve  —  The 
Effect  of  Changes  in  Slope  —  Rate  of  Transmission  of  the  Flood 
Wave  —  Flood  Prediction  by  Measuring  Rainfall  —  Variability  of 
the  Rainfall  —  Computations  of  the  Run-off  —  Variations  in  Dif- 
ferent Months  —  Prediction  of  Floods  in  the  Department  of 
Ardeche,  France  —  Flood  Prediction  by  Discharge  Measurement  — 
.  Method  Employed  on  the  Elbe  River  —  Flood  Prediction  by  Gauge 
Relations  —  Method  Employed  on  the  Rhine  River  —  Method 
Employed  on  the  Seine  River  —  Combinations  of  the  Methods  of 
Belgrade  and  Von  Tein  suggested  for  the  Rivers  of  the  United 
States. 

CHAPTER  VI 

RIVER  REGULATION        .........  44-54 

Methods  Proposed  for  Improving  River  Channels  —  The  Neces- 
sity for  Obtaining  Increased  Depths  in  Rivers  —  Parallel  Longitudi1 
nal  Dikes  —  River  Straightening  —  The  French  Method  of  River 
Regulation  —  M.  Fargues'  Laws  —  The  Proper  Curve  to  be  Given 
the  Channel  —  Discussion  of  the  Effect  of  Straight  Reaches  on  the 
Shape  of  the  Bars  —  The  Height  of  Jetties  and  Dikes  —  River 
Improvement  not  Susceptible  of  Rigid  Mathematical  Analysis  — 
The  River  Rhine  Improved  by  Straightening. 

CHAPTER  VII 

DIKE  CONSTRUCTION  AND  BANK  PROTECTION  ....  55-66 

Requirements  of  the  French  System  of  River  Regulation  —  A 
Rigid  Adherence  Not  Necessary  in  the  United  States  —  The  Longi- 
tudinal Dike  Replaced  by  Bank  Revetment  —  Permeable  Dikes 
Substituted  for  Those  of  Stone  and  Gravel  —  Danger  of  Destruc- 
tion of  Permeable  Dikes  by  Drift  and  Ice  —  Retards  —  Where 
Material  in  Suspension  is  Small,  Dikes  of  Stone  and  Brush  Neces- 
sary —  Dike  Construction  on  the  Rhine  —  The  Proper  Location 
and  Inclination  of  Spur-Dikes  —  French  and  Italian  Practice  —  Ad- 
vantages of  Permeable  Dikes  on  the  Concave  Bank  of  a  River  — 
Method  of  Bank  Protection  Generally  Adopted  in  Europe  —  The 
Blees  Werke  of  the  Rhine  —  The  Triangle  Werke  —  Channel 
Straightening  on  the  Rhine  as  a  Method  of  Protecting  Banks  — 
Noblings  —  Bank  Revetment  in  the  United  States  —  Concrete 
Mats  —  Precautions  to  be  Observed  in  Mattrass  Construction  — 
Substitutes  Suggested  for  Bank  Protection. 

CHAPTER  VIII 

RIVER  IMPROVEMENT  BY  CANALIZATION  ,         .         .         .         .  67-88 

The  Effect  of  a  Dam  on  a  River's  Regimen  —  The  Substitution 
of  Movable  Dams  —  The  Types  of  Movable  Dams  —  Dams  Rarely 
Constructed  of  a  Single  Type  —  The  Drowning  Out  of  Dams  —  The 


CONTENTS  Vll 

PAGES 

Types  Ordinarily  Used  in  the  United  States  —  The  Substitution  of 
Concrete  and  Metal  for  Wood  in  Dam  Construction  —  Increase  in 
the  Heighth  of  Dams  Due  to  the  Development  of  Electrical  Power 

—  Effect  on  the  Stability  of  the  Structures  —  Foundations  of  Rock, 
Gravel,  Sand  and  Clay  —  Methods  of  Reducing  Percolation  Under 
Dams  —  Protection  from  Overflow  —  Protection  of  Banks  from 
Percolation   and   Eddy  Action  —  The  Location  of  Dams  —  The 
Lateral  Canal  —  The  Flow  of  Sediment  in  Pools  —  A  River  Flowing 
through  Glacial  Drift  More  Readily  Improved  by  Canalization 
Than  One   Flowing  in  an  Alluvial  Valley  —  The  St.   Lawrence 
Valley  versus  the  Mississippi  —  Canalization  versus  Regulation  — 
The  Lateral  Canal  versus  a  Canalized  Bed  —  The  Reduction  of 
Reservoir  Capacity  by  the  Deposition  of  Sediment  —  Methods  of 
Removing  the  Deposition  —  The  Sirhind  Irrigation  Canal  in  India 

—  Inclines,  Lifts  and  Locks  —  The  Lock  —  The  Lock  Gate  —  Re- 
pairs to  Lock  Gates  and  Their  Protection  —  Filling  and  Emptying 
Locks  —  The  Width  of  Locks. 

CHAPTER  IX 

DREDGING,  REMOVAL  OP  OBSTRUCTIONS,  BUOYS  AND  LIGHTS    .         .  89-97 

A.  DREDGING    .......:..  89-93 

Dredging  in  Valleys  Formed  of  Glacial  Drift  —  The  Improvement 
of  Rivers  Emptying  Into  the  Great  Lakes  —  The  Lowering  of  Pools 
from  Dredging  —  Dredging  in  Canalized  Rivers  Below  Dams  —  The 
Limitations  of  Dredging  in  Rivers  Carrying  Large  Amounts  of  Sedi- 
ment —  Dredging  Required  When  it  is  Attempted  to  Straighten  a 
River  by  Regulation  —  Rate  of  Movement  of  Sand  Waves  Through 
Straight  Reaches  —  The  Adaptability  of  Dredges  to  Various  Kinds 
of  Work  —  Substitutes  Proposed  for  Dredging. 

B.  ROCK  EXCAVATION         .         .        ..         .         .         .         .         .  94-95 

By  Coffer  Dam  —  By  Lobnitz  Crusher  — The  Diving  Bell  — 
Drill  Barges  —  Rock  Removal  at  Hell  Gate,  New  York  Harbor  and 
Blossom  Rock,  San  Francisco  Harbor. 

C.  REMOVAL  OF  SNAGS      .         .         .         .         .         .         .         .  95-96 

The  Snag  Boat  —  The  Removal  of  Trees  from  Caving  Banks  — 
The  Removal  of  Rafts  from  the  Atchafalaya  and  Red  Rivers. 

D.  BUOYS,  LIGHTS  AND  BEACONS        .         .         .         .         .         .  96-97 

The  Engineer  Department  Assists  the  Bureau  of  Lighthouses  in 
Light  and  Buoy  Location  on  the  Western  Rivers  of  the  United 
States  —  Methods  of  Marking  a  Channel  —  Lighthouses  —  Buoys 

—  Auxiliary  Aids. 

CHAPTER  X 

RESERVOIRS  AND  LEVEES  AS  A  MEANS  OF  IMPROVING  NAVIGATION  .  98-106 
RESERVOIRS      <         .         .         .     •    ,         .         .      '•• :         .         .  98-102 
The  Effect  of  Increasing  the  River's  Discharge  on  Depths  in  Pools 


Vlll  CONTENTS 

PAGES 

and  Over  Bars  —  The  Effect  of  a  Constant  Flow  at  a  Given  Stage  — 
The  Changes  in  Depth  in  the  Mississippi  River  as  Its  Discharge 
Increases  —  Reservoir  Construction  at  the  Head  Waters  of  the  Mis- 
sissippi River  —  Their  Influence  on  Navigable  Depths  —  The  Dif- 
ficulties in  Practically  Manipulating  Reservoirs  —  General  Rules 
Observed  in  Operating  the  Reservoirs  on  the  Upper  Mississippi 
River. 

IMPROVING  THE  Low  WATER  CHANNEL  BY  CONFINING  THE  FLOOD  DIS- 
CHARGE BETWEEN  LEVEES 102-106 

The  Force  Generated  During  Floods  —  Its  Relation  to  Useful 
Work  —  Difficulty  of  Constructing  Levees  so  as  to  Force  the  Flood 
Discharge  to  Follow  the  Low  Water  Channel  —  The  Use  of  Levees 
to  Improve  Navigation,  Corollary  of  River  Straightening  —  Euro- 
pean Practice  —  Improvement  of  the  Danube  River  —  Comparison 
of  the  Improvement  of  the  Danube  and  Rhone  Rivers. 

CHAPTER  XI 

FLOOD  PROTECTION     .........  107-124 

The  Area  of  Erosion  —  Forest  Growth  —  Terraces  —  The  Zone  of 
Deposition  —  Levees  —  Retaining  Dams  —  Hydraulic  Mining  on 
the  Tributaries  of  the  Sacramento  River  —  The  Raising  of  the 
River  Bed  from  Deforestation  and  Drainage  —  The  Difficulties  of 
the  Problem  —  The  Claims  of  M.  Proney  and  Herr  Wex  —  Recent 
Investigations  —  The  Yellow  River  of  China  —  The  Protection  of 
the  Portion  of  a  Valley  where  Deposition  and  Fill  are  in  Unstable 
Equilibrium  —  Mounds  —  Forest  Growth  —  Storage  Reservoirs  — 
Retarding  Basis  —  Enlargement  of  the  River  Section  —  River 
Straightening  and  Cut-Offs  —  Outlets  —  Waste  Weirs  —  Improve- 
ment of  the  Sacramento  River  —  The  Junction  of  the  Red, 
Atchafalaya  and  Mississippi  Rivers  —  Levees  —  Their  Form  and 
Dimensions  —  Methods  Employed  in  Levee  Construction  —  Flood 
Protection  of  the  Miami  River  —  The  Proper  Method  of  Treat- 
ment of  the  Flood  Waters  of  a  Large  River  Basin. 

CHAPTER  XII 

ESTUARIES 125-132 

The  Tidal  Flow  —  The  Influence  of  the  Form  of  the  Estuary  on 
the  Tidal  Flow  —  Tidal  Propogation  —  The  Bore  —  The  Effect  of 
Tidal  Oscillation  on  the  Flow  of  Rivers  —  The  Movement  of  Silt  in 
Estuaries  —  The  Difference  in  the  Principles  Governing  the  Im- 
provement of  the  Tidal  and  Non-Tidal  Portions  of  a  River  —  The 
Objections  to  the  Curved  Trace  in  Estuaries — Removal  of  Barriers 
to  the  Tidal  Flow  —  The  Width  of  the  Tidal  Section  —  Longitudi- 
nal versus  Spur  Dikes  —  Dredging  —  The  Improvement  of  the 
River  Clyde. 


CONTENTS  IX 

CHAPTER  XIII 

PAGES 

THE  MOUTHS  OF  RIVERS    .         .         .         .         .         *         .         .   133-144 

Ocean]  Waves  —  Their  Height  —  Their  Oscillation  —  Their 
Form  —  Their  Force  —  Damage  Caused  to  Breakwaters  —  Damage 
to  Cliffs  —  Movement  of  Material  by  Wave  Action  on  a  Beach  — 
Effect  of  an  Obstacle  on  the  Flow  of  Sediment  —  The  Deposition  at 
the  Mouths  of  Rivers  —  Deltas  —  The  Improvement  of  Deltas  — 
The  Danube  and  the  Mississippi  Rivers  —  The  Rhone  River  — 
Rivers  whose  Outlets  Maintain  Themselves  —  The  Improvement 
of  the  Entrance  to  New  York  Harbor  —  Tidal  Rivers  Improved  by 
Parallel  Jetties  —  Sluicing  Basins  —  The  Malamocco  Entrance  to 
the  Lagoon  of  Venice  —  Converging  Dikes  —  The  Employment  of  a 
Single  Jetty  —  The  Improvement  of  the  Mouth  of  Cape  Fear  River 

—  The  Single  Jetty  at  Sandusky  Harbor  —  The  Reaction  Break- 
water —  Wave  Action  Less  Intense  Against  Jetties  on  River  Bars 
than  Against  Harbor  Breakwaters  —  Mattrass  Foundations. 

CHAPTER  XIV 

HARBORS   145-153 

The  Location  of  Harbors  —  Essential  Elements  of  Harbor  Con- 
struction —  Harbor  Entrances  —  Their  Proper  Form  to  Produce  the 
Greatest  Tranquillity  —  The  Width  of  Entrance  —  The  Exterior 
Breakwaters  Employed  on  the  Great  Lakes  to  Reduce  Wave  Action 
within  the  Harbor  —  Stilling  Basins  —  Rubble  Mound  Breakwaters 

—  Effect  of  Waves  on  their  Sea  Slopes  —  The  Paving  of  the  Sea 
Slope  —  The  Substitution  of  Stone  or  Concrete  in  Large  Masses  for 
the  Pavement  —  General  Dimensions  of  Breakwaters  in  the  United 
States  —  Breakwater  Construction  Dependent  on  the  Character  of 
the  Quarry  from  which  the  Stone  is  Derived  —  The  Unpaved 
Breakwater  More  Effective  in  Dissipating  the  Wave  Energy  — 
Vertical  Breakwaters  —  Causes  of  Failure  of  Early  Masonry  Types 

—  Advantages  in  the  Substitution  of  Concrete  for  Masonry  —  Verti- 
cal Breakwaters  on  Rubble  Mounds  —  The  Failure  of  the  Alderney 
Breakwater  and  that  at  Sandy  Beach  Harbor  of  Refuge,  Mass. 

—  The  Necessity  of  Protecting  the  Vertical  Breakwater  on  a  Vertical 
Mound  by  Wave  Breakers  of  Concrete  Blocks  —  The  Timber  Crib 
Breakwater  of  the  Great  Lakes  —  The  Necessity  for  Tidal  Basins  in 
Europe  —  The  Advantages  of  the  Piers  Employed  in  the  United 
States. 

CHAPTER  XV 

THE  ECONOMIES  OF  WATER  TRANSPORTATION       ...         .         .   154-166 

The  Determination  of  the  Worthiness  of  an  Improvement  Re- 
quired Before  Final  Legislative  Action  —  Economies  of  River 
Navigation  with  Animal  Traction  —  Economies  Produced  by  the 
Introduction  of  the  Locomotive  —  The  Resistance  to  Motion  of 
Boat  and  Car  —  Economies  Resulting  from  Enlarging  Car  and 


CONTENTS 

FAQ] 

Boat — The  Compound  Condensing  Marine  Engine  versus  The  Non- 
Condensing  Locomotive  —  Fuel  Economy,  and  Labor  Expenditure 
per  ton  Mile  on  Rail  and  River  —  Overhead  Charges  —  Relative 
Costs  of  Constructing  and  Maintaining  Rivers  and  Railroads  — 
Causes  of  Decline  in  Commerce  on  Western  Rivers  —  Car  Ferries  — 
Influence  of  East  and  West  Movement  of  Freight  in  the  United 
States  on  Transportation  by  Rail  and  Water  —  Recent  Legislative 
Enactment  Beneficial  to  Water  Transportation  —  The  Effect  of 
Delays  in  Loading  and  Unloading  on  the  Cost  of  Transportation  — 
The  Economic  Limitations  to  the  Size  of  Vessels  —  The  Relation  of 
Density  of  Traffic  to  Ocean  Transportation  —  The  Dimensions  of 
Piers  —  The  Pier  Shed  —  Railroad  Tracks  on  Piers  —  The  Effect  of 
Tidal  Fluctuation  and  Flood  Heights  on  Loading  Vessels. 


APPENDIX  A       .  .         .......  167-172 

Bibliographic  Notes. 

APPENDIX  B 173-179 

Flood  Prediction  at  the  Mouth  of  the  Ohio  River  —  Flood  Pre- 
diction at  the  Mouth  of  the  Missouri  River  —  Flood  Prediction  at 
the  Mouth  of  the  White  River.  (See  also  TABLES.) 

APPENDIX  C.     THE  INFLUENCE  OF  FOREST  ON  STREAMS       .         .  180-182 

Report  of  Meteorological  Section  of  the  Experimental  Depart- 
ment of  Forestry  of  Germany. 

TABLES 183-189 

Table  I.  Computation  of  the  Ohio  River  Floods  at  Cairo  —  Table 
II.  Computation  of  Mississippi  River  Floods  at  Cairo  —  Table  III. 
Computation  of  Missouri  River  Floods  at  St.  Louis  —  Table  IV. 
Computation  of  Mississippi  River  Floods  at  the  Mouth  of  White 
River. 


RIVER  AND  HARBOR  CONSTRUCTION 


CHAPTER  I 
INTRODUCTION 

During  his  professional  career  the  writer  has  prepared  numerous 
projects,  and  has  answered  many  criticisms  of  the  methods  em- 
ployed in  the  improvement  of  rivers  and  harbors.  The  following 
pages  are  derived  principally  from  these  sources.  In  replying  to 
complainants,  the  author  came  to  the  conclusion  that  there  was 
a  commendable  interest  among  the  American  people  in  the  sub- 
ject of  the  improvement  of  rivers  and  harbors,  and  a  deplorable 
ignorance  of  the  fundamental  principles  governing  the  flow  of 
water  in  natural  channels.  Such  is  his  apology  for  this  pub- 
lication. 

The  ordinary  textbook  on  hydraulics  treats  principally  of  the 
flow  of  water  in  pipes  and  conduits,  and  the  ordinary  engineer  is  apt 
to  consider  a  river  as  merely  a  large  conduit  governed  by  the  same 
laws,  ignoring  the  change  in  conditions  arising  from  the  fact  that 
the  walls  of  a  pipe  are  not  affected  by  the  velocity  of  flow  through 
it,  while  the  channels  of  a  silt-bearing  stream  expand  or  contract 
with  every  change  in  the  volume  of  discharge.  This  fact  is  ig- 
nored also  by  many  writers  on  river  hydraulics,  whose  textbooks 
contain  more_or  less  elaborate  discussions  of  the  formulas  Q  =  VA, 
and  V  =  C\/RS,  which  are  authoritatively  stated  as  guides  for  de- 
termining the  depth  in  a  contracted  waterway. 

In  a  rock  cut,  these  formulas  give  as  accurate  results  as  they  do 
when  applied  to  a  sewer  or  to  a  pipe,  but  in  the  alluvium  of  the 
Mississippi  or  Missouri  rivers,  scour  caused  by  the  contraction 
produces  a  radical  change  in  the  hydraulic  radius.  Water  flow- 
ing in  a  river  channel  is  governed  by  the  same  immutable  laws 
as  when  flowing  in  pipes,  but  these  laws  are  so  modified  in  the  river 
by  those  governing  the  flow  of  sediment  that  effects  are  produced 
which  are  erroneously  termed  exceptions  to  general  laws. 

For  example  if  a  given  pipe  is  replaced  by  one  of  less  diameter, 
the  head  (slope)  must  be  increased  to  obtain  the  same  discharge. 

1 


2  IL'iU        RIVERS   AND   HARBORS 

"         *"cClv>*>  *«,"•"***  *          °          *'       *• 

In  ii  stream  ^with,  a  mobile  bed,  however,  if  the  channel  is  con- 
tracted and  at  the  same  time  straightened,  the  resultant  increase 
in  velocity  produces  a  scour  which  enlarges  the  hydraulic  radius 
to  such  an  extent  as  to  reduce  the  slope  through  the  contracted 
section. 

In  the  discussion  of  hydraulic  problems,  there  is  too  great  a 
tendency  to  draw  conclusions  from  special  cases  without  taking 
into  consideration  all  the  conditions  which  surround  them.  Thus, 
if  one  side  of  a  hill  is  covered  with  trees  and  the  other  side  is  cul- 
tivated, the  rapidity  with  which  the  snow  melts  is  attributed  to 
the  forest  growth,  without  taking  into  consideration  the  fact  that 
one  may  have  a  southern  exposure  and  the  other  a  northern, 
and  that  the  inclination  of  the  sun's  rays  to  the  surface  of  the 
ground  may  have  a  greater  effect  in  melting  the  snow  than  the 
surface  covering.  As  another  illustration,  if  one  year's  flood, 
attaining  a  height  X  at  a  locality  A,  produced  a  height  Y  at  a 
point  B  further  down  a  river,  it  often  has  been  assumed  that  upon 
a  repetition  of  the  height  X  at  A,  the  height  F  would  recur  at 
B  unless  there  had  been  an  enlargement  or  diminution  of  the 
river  section  between  the  two  localities,  thereby  ignoring  the 
influence  of  the  discharge  of  the  tributaries  below  B  on  the  river's 
regimen. 

While  harbors  were  improved  before  the  Christian  Era,  and 
canals  were  constructed  in  the  Middle  Ages,  the  improvement  of 
rivers  is  of  recent  origin,  and  owes  its  development  to  the  invention 
of  the  steamboat.  The  early  efforts  to  regulate  rivers  were  gen- 
erally unsuccessful,  and  it  is  only  within  the  past  thirty  years  that 
the  correct  principles  of  river  regulation  have  been  evolved, 
principally  by  French  and  German  engineers. 

There  is  a  strong  temptation  for  the  author  of  a  textbook  to 
compile  data  from  the  works  of  earlier  authors.  An  American 
unfamiliar  with  foreign  languages  is  practically  limited  in  his 
knowledge  of  European  practice  in  river  improvement  to  transla- 
tions of  certain  French  and  German  works  made  by  officers  of 
the  Corps  of  Engineers,  U.  S.  Army,  forty  or  fifty  years  ago, 
which  are  now  out  of  print,  and  to  the  proceedings  of  the  various 
navigation  congresses.  Some  of  our  textbooks  therefore  quote 
with  approval  methods  of  river  regulation  long  since  abandoned 
by  the  nations  in  which  they  originated.  For  example,  the  proj- 
ect of  1882  for  improving  the  Danube  river,  though  vitally  modi- 


INTRODUCTION  3 

fied  by  the  Austrian  Government  in  1899,  is  still  given  as  an  illus- 
tration of  the  proper  method  of  improving  a  river. 

As  this  book  has  been  derived  principally  from  reports  and 
addresses  in  advocacy  of  certain  propositions,  the  author  recog- 
nizes that  he  is  prone  to  discuss  a  subject  as  a  lawyer  would  pre- 
sent the  evidence  before  a  jury,  and  that  he  may  at  times  give  too 
great  weight  to  his  own  views  instead  of  judicially  summing  up 
the  concensus  of  opinion  among  engineers.  He  has  also  failed  to 
give  proper  credit  to  the  numerous  authors  from  whom  he  has 
derived  his  ideas.  He  makes  no  claim  to  originality,  but  his 
notes  extend  over  an  active  professional  life  of  forty  years,  and 
he  has  now  forgotten  the  sources  of  much  of  his  information. 

De  Mas'  Rivieres  a  Courant  Libre,  and  Harcourt's  Rivers  and 
Canals  and  Harbours  and  Docks,  are  the  foundations  of  the  struc- 
ture; Jasmund's  Die  Arbeiten  der  Rheinstrom-Bauverwaltung ,  and 
the  Report  of  the  Italian  Commission  appointed  in  1903  to  in- 
vestigate the  internal  navigation  of  the  valley  of  the  Po,  have  been 
quoted  freely.  The  student  will  also  recognize  extracts  from  Van 
Ornum's  Regulation  of  Rivers,  Thomas  and  Watts'  Improvement  of 
Rivers,  Shield's  Principles  and  Practice  of  Harbour  Construction, 
and  Wheeler's  Tidal  Rivers.  The  ANNALES  DBS  FONTS  ET  CHAUS- 
SEES,  the  transactions  of  the  various  engineering  societies,  the 
PROCEEDINGS  OF  THE  PERMANENT  ASSOCIATION  OF  NAVIGATION 
CONGRESSES,  THE  PROFESSIONAL  MEMOIRS,  CORPS  OF  ENGI- 
NEERS, U.  S.  A.,  and  the  REPORTS  OF  THE  CHIEF  OF  ENGINEERS, 
U.  S.  ARMY,  are  mines  of  information  on  river  and  harbor  con- 
struction. 

While  a  proper  conception  of  the  theory  of  engineering  con- 
struction is  necessary,  a  knowledge  of  existing  practice  is  also 
indispensable.  In  river  and  harbor  construction,  every  engineer- 
ing district  affords  data  for  an  extensive  treatise  on  that  subject, 
and  there  is  urgent  need  of  a  condensed  work  on  American  prac- 
tice in  the  improvement  of  estuaries,  of  the  mouths  of  rivers,  and 
of  harbors,  similar  to  Van  Ornum's  book  on  River  Regulation, 
and  Thomas  and  Watts'  chapters  on  lock  and  dam  construction 
in  their  book  on  Improvements  of  Rivers. 

A  statement  of  the  principles  of  river  hydraulics  within  the 
limits  of  a  single  volume  has  required  intense  condensation  and 
to  attempt  to  illustrate  the  applications  of  these  principles  in 
practice  would  require  a  large  addition  to  its  pages.  By  request, 


4  RIVERS   AND   HARBORS 

notes,  indicated  in  the  text  by  numbers,  have  been  appended  which, 
while  not  attempting  to  give  a  complete  bibliography,  will  show 
the  student  where  he  can  find  practical  applications  of  the  prin- 
ciples discussed,  and  the  pros  and  cons  of  subjects  which  may  have 
been  too  positively  stated  in  the  text.  These  notes  are  of  more 
value  to  officers  of  the  Corps  of  Engineers  and  others  who  have 
access  to  the  library  of  the  Engineers  School  of  the  U.  S.  Army, 
or  to  the  Congressional  Library  at  Washington,  D.C.,  than  to 
the  general  reader,  as  the  writer  has  found  by  experience  that, 
ordinarily,  libraries  are  limited  in  volumes  on  river  hydraulics 
to  the  textbooks  quoted  above. 


CHAPTER  II 
THE  FORMATION  OF  RIVERS 

The  origin  of  the  water  supply,  of  rivers  and  streams,  is  the 
ocean  into  which  they  in  turn  discharge.  Aqueous  vapor  evapo- 
rated from  its  surface  is  carried  by  the  winds  over  the  land,  where 
it  condenses  and  is  deposited  as  rain  or  snow.  A  certain  portion 
of  this  precipitation  returns  to  the  sea  over  the  surface  of  the 
ground,  while  another  part  is  absorbed  by  the  soil,  and  after  a 
subterranean  flow  of  variable  duration  appears  on  the  earth's 
surface  in  springs. 

In  this  general  flow  of  moisture  from  ocean  to  land  and  return, 
there  are,  however,  numerous  short  circuits.  Evaporation  occurs 
not  only  from  the  surface  of  rivers  and  streams,  but  also  from 
the  soil  itself,  and  in  such  quantities  during  summer  months  as 
to  materially  reduce  the  surface  flow.  Over  large  lakes,  the 
evaporation  may  be  sufficient  to  cause  a  local  precipitation  along 
their  borders.  The  roots  of  trees  and  of  other  vegetation,  under 
certain  conditions,  extract  a  large  percentage  of  the  water  ab- 
sorbed by  the  soil,  and  return  it  to  the  atmosphere  through  their 
leaves,  thus  reducing  the  subterranean  flow  of  streams.  Also  at 
the  mouths  of  rivers  there  is  frequently  a  large  ebb  and  flow  of 
salt  water  from  the  sea,  due  to  tidal  influences. 

The  ratio  of  surface  flow  to  absorption  is  dependent  on  the 
permeability  of  the  soil,  the  surface  covering,  its  surface  slope, 
and  the  intensity  and  duration  of  the  rainfall.  A  sandy  soil 
absorbs  more  water  than  one  whose  principal  ingredient  is  clay, 
and  even  rock,  especially  limestone,  frequently  contains  permeable 
seams  and  crevices,  which  permit  a  large  subterranean  flow.  A 
sandy  soil  in  summer  may  absorb  a  rainfall  which  in  early  Spring 
would  have  flowed  off  its  surface,  due  to  its  frozen  condition. 

In  forests,  the  decayed  leaves  and  mosses  create  a  humus  which 
will  absorb  a  large  amount  of  water,  but  many  kinds  of  vegeta- 
tion produce  roots  which  seriously  interfere  with  the  flow  of  such 
waters  in  the  underlying  soil,  retaining  it  in  the  humus  as  in  a 
reservoir.  The  roots  of  Bermuda  grass  form  a  more  impermeable 

5 


6  RIVERS   AND   HARBORS 

covering  than  those  of  wheat,  corn  or  cotton.  A  root  of  an  Osage 
orange  tree  may  extend  a  long  distance  in  a  horizontal  direction 
in  its  search  for  moisture  and  open  up  a  sub-surface  channel 
around  it  which  ultimately  may  be  destructive  to  a  levee;  while 
roots  of  other  species  of  forest  growth  may  retard  such  flow. 

The  surface  slope  affects  the  degree  of  saturation  of  the  soil. 
A  hillside  retains  less  water  than  a  plain,  since  both  the  surface 
flow  and  the  subterranean  flow  are  accelerated  by  the  slope. 
Even  with  porous  soils,  there  is  a  period  during  every  rainfall 
when  the  precipitation  exceeds  the  capacity  of  the  soil  to  absorb 
it  and  of  the  subterranean  channels  to  remove  it;  the  surplus 
water  then  flows  on  the  surface. 

The  surface  flow  is  therefore  a  function  of  the  intensity  of  the 
precipitation,  the  slope  of  the  ground,  and  the  roughness  of  its 
surface  covering.  During  a  light  rainfall,  in  a  plowed  field,  the 
degree  of  saturation  and  the  velocity  of  the  surface  discharge  is 
affected  by  the  direction  of  the  furrows.  Vegetation  materially 
retards  the  flow.  In  forests,  fallen  trees,  branches,  and  brush 
may  collect  in  heaps  which  create  timber  dams,  similar  to  the 
rafts  which  formerly  obstructed  some  of  our  western  rivers. 

A  heavy  precipitation  on  a  steep  mountain  slope  produces  such 
a  volume  and  velocity  of  discharge  as  to  create  a  powerful  erosive 
action  which  not  only  removes  particles  of  soil  and  its  vegetable 
covering  but,  when  concentrated  in  the  channels  of  ravines,  can 
move  large  boulders  and  forest  growth.  Every  hillside  is  being 
degraded  by  this  force,  and  it  is  assisted  in  its  destructive  work 
by  frost,  which  disintegrates  rock  masses  and  renders  them  sus- 
ceptible to  its  action.1 

The  material  eroded  from  the  hills  is  transported  as  far  as  the 
waters  which  dislodged  it  maintain  the  velocity  they  originally 

1  Aa  an  example  of  the  tran3portive  force  of  rapidly  moving  water,  the  following  personal 
experience  of  the  writer  may  be  cited.  He  was  requested  by  the  Insular  Government  of  the 
Philippines  to  inspect  a  road  which  had  just  been  completed  in  the  valley  of  the  Bug  River, 
which  flows  in  the  mountainous  regions  of  the  province  of  Benguet,  Island  of  Luzon.  At  a 
certain  locality,  in  order  to  form  a  shelf  on  which  to  construct  the  roadway,  a  large  amount  of 
rock  had  been  blasted  from  the  mountain  side  and  had  fallen  into  the  valley  below.  At  the 
the  time  of  the  inspection,  the  channel  of  the  river  was  so  choked  with  debris  that  its  flow,  then 
insignificant,  was  through  the  dump  pile. 

A  few  days  afterward  a  rainfall  of  eighteen  inches  occurred  in  the  valley,  most  of  the  pre- 
cipitation occurring  within  twenty-four  hours.  Three  days  after  the  storm,  the  writer  made  a 
return  trip  over  the  remnants  of  the  road.  The  Bug  River  was  again  an  insignificant  stream, 
but  the  rock  pile  had  disappeared,  and  the  river  was  placidly  flowing  in  its  original  bed.  The 
debris  was  scattered  through  the  lower  valley,  masses  of  rock  weighing  at  least  ten  tons  having 
been  transported  long  distances. 


THE   FORMATION   OF   RIVERS  7 

acquired,  but  in  the  channels  of  streams  flowing  from  a  mountain 
peak  to  the  sea  the  slope  of  the  earth's  surface  rapidly  diminishes. 
At  first  the  concentration  of  the  flow  in  gullies  and  ravines,  by 
reducing  the  frictional  resistance,  will  maintain  the  velocity  ac- 
quired on  the  steeper  slope,  but  as  the  ravine  widens  into  a  valley 
the  carrying  capacity  of  its  waters  is  diminished  and  a  deposition 
of  the  material  transported  occurs.  With  a  reduction  in  the 
velocity,  the  boulders  are  first  deposited,  then  successively  smaller 
stones,  gravel,  and  the  heavier  sands.  The  finer  sands  and  clays 
are  frequently  transported  long  distances,  a  sufficient  reduction 
in  velocity  not  occurring  to  cause  their  deposition  until  the  stream 
empties  into  a  lake  or  sea. 

Since  the  velocity  of  flow  varies  in  different  parts  of  the  cross- 
section  of  a  torrential  stream,  as  in  other  channels,  finer  material 
is  deposited  intermingled  with  the  boulders.  A  boulder  once  at 
rest  induces  whirls  and  eddies  which  cause  the  settlement  around 
it  of  sand  and  gravel,  imbedding  it  in  the  channel  so  that  a  flood 
of  greater  intensity  or  longer  duration  is  necessary  to  set  it  in 
motion  again.  In  the  disintegration  of  stratified  rock  masses,  the 
fragments,  even  those  of  the  size  of  gravel,  are  in  the  form  of 
slabs  when  they  are  first  detached,  and  they  may  be  deposited 
overlapping  one  another  so  as  to  form  a  pavement  which  may 
oppose  a  great  resistance  to  erosion  so  long  as  the  current  main- 
tains the  direction  that  caused  the  deposition,  yet  a  change  in  the 
direction  of  the  flow  attacking  these  slabs  from  the  side  instead 
of  on  the  surface  may  destroy  the  pavement.  For  example,  a 
gravel  bar  in  a  river  may  resist  the  flow  of  floods  for  ages,  but  if 
a  dam  is  constructed  on  such  a  foundation,  the  percolation  under 
it  may  first  remove  particles  of  sand  and  clay  mixed  with  the 
gravel  and  thus  create  a  channel  through  which  the  velocity 
of  the  discharge,  though  less  than  that  of  an  unobstructed  flood, 
may  dislodge  the  gravel  by  reason  of  the  change  in  direction 
of  its  attack. 

The  imbedding  of  detritus  in  stream  channels  tends  to  reduce 
the  area  of  deposition  of  the  coarser  materials  to  narrow  limits. 
But  as  rock  masses  roll  down  a  stream  they  impinge  upon  one 
another,  producing  a  grinding  effect  which  reduces  the  dimensions 
of  the  boulders,  breaks  the  slabs  into  smaller  pieces,  converts 
stone  into  gravel  and  gravel  into  sand,  and  tends  to  give  a  spherical 
form  to  all  the  elements  set  in  motion.  This  facilitates  the 


8  RIVERS   AND   HARBORS 

movement  and  causes  a  gradual  reduction  in  the  size  of  the  ele- 
ments which  form  the  deposits,  the  farther  the  debris  is  trans- 
ported from  the  zone  of  degradation.  Since  material  rolling 
along  a  stream  bed  has  a  smaller  velocity  than  the  current,  and 
since  a  heavy  precipitation  is  of  short  duration,  the  movement 
of  such  material  is  intermittent  and  the  distance  traveled  is 
relatively  short  during  each  storm;  but  material  fine  enough  to 
be  carried  in  suspension  moves  with  the  velocity  of  the  stream, 
and  is  less  subject  to  this  intermittent  deposition. 

The  subterranean  flow  of  waters  is  slow  except  through  rock 
crevices,  due  to  the  obstructions  which  particles  of  the  soil  offer 
to  its  passage.  It  is  governed  by  the  same  laws  as  is  other  flowing 
water,  its  velocity  being  a  direct  function  of  the  head  and  an 
inverse  function  of  frictional  resistance.  However,  as  will  be 
explained  in  discussing  the  laws  governing  the  flow  of  water,  the 
ordinary  hydraulic  formulas  are  inapplicable  to  subterranean 
flow,  for  it  has  been  found  by  experiment  that  the  velocity  varies 
approximately  as  the  head  instead  of  as  the  square  root  of  the 
head.  There  is  also  a  great  diminution  in  the  velocity, — that  in 
pipes  for  instance  being  ordinarily  measured  in  feet  per  second, 
whereas  that  in  ordinary  soil  is  measured  in  feet  per  day. 

Since  the  permeable  crust  of  the  earth  is  of  vast  extent  and  is 
frequently  of  great  depth,  there  exists  an  underground  water- 
system  of  streams,  rivers,  and  lakes  similar  to  those  which  appear 
on  the  surface,  but  these  underground  streams  flow  with  very 
much  smaller  velocity.  Wherever  the  surface  of  the  ground 
intersects  the  surface  of  one  of  these  subterranean  streams,  springs 
appear.  These  springs  increase  the  discharge  of  a  river  at  all  stages, 
and  they  are  its  principal  sources  of  supply  during  low  water. 
If  this  intersection  takes  place  in  the  bed  of  a  stream,  the  differ- 
ence of  head  of  the  underground  waters  and  of  the  surface  waters 
determines  the  rate  of  discharge;  and  when  the  head  of  the 
surface  water  is  the  greater,  the  surface  stream  may  even  dis- 
appear and  be  absorbed  in  the  sub-surface  flow.  The  failure  to 
recognize  this  fundamental  principle  has  caused  large  errors  in 
estimating  the  capacity  of  reservoirs  to  store  water  and  of  drain- 
age ditches  to  discharge  it. 

Variations  in  precipitation  cause  a  rise  and  fall  of  the  surface  of 
these  underground  waters,  similar  to  the  floods  in  rivers.  On  account 
of  the  sluggishness  of  the  flow  and  the  great  reservoir  capacity  of 


THE    FORMATION    OF   RIVERS  9 

the  permeable  strata,  a  long  period  of  time  may  elapse  before  these 
changes  manifest  themselves  on  the  earth's  surface.  For  example, 
at  the  head  waters  of  the  Mississippi  River,  a  yearly  minimum 
rainfall  does  not  produce  a  minimum  stream  flow  until  the  low 
water  season  of  the  year  succeeding  its  occurrence. 

The  water  which  filters  through  soils  for  long  distances  carries 
little  material  in  suspension,  but  it  may  contain  a  large  amount 
of  soluble  matter,  which,  by  evaporation  of  the  water,  may 
produce  deposits  of  considerable  extent.  If  the  head  of  the  sub- 
surface flow  exceeds  one-tenth  of  the  distance  it  travels,  however, 
the  water  may  acquire  sufficient  force  to  remove  particles  of  the 
soil,  which  is  an  important  consideration  in  designing  levees  and 
other  structures  whose  foundations  rest  on  permeable  material. 

While  subterranean  flow  is  the  principal  source  of  a  river's 
water  supply  during  low  water,  the  higher  stages  of  the  river 
are  created  by  the  addition  thereto  of  surface  flow.  Hence  a 
river's  stage  becomes  a  function  of  the  relative  amount  of  the 
precipitation  that  is  absorbed  by  the  ground  and  by  the  surface 
flow,  but  it  is  modified  by  the  reservoir  capacity  of  the  channel, 
and  by  evaporation.  A  lake  tends  to  diminish  a  river's  maximum 
discharge  and  to  increase  the  minimum  discharge.  If  the  pre- 
cipitation is  greater  during  the  winter  and  early  spring  than 
during  the  summer  and  early  fall,  evaporation  will  diminish  the 
low  water  flow.  But  in  localities  where  the  conditions  are  re- 
versed, evaporation  will  reduce  the  discharge  during  the  summer 
high  stages,  particularly  where  reservoirs  expose  a  large  surface 
to  the  action  of  winds  and  of  the  sun's  rays. 

During  extreme  low  stages,  the  velocity  of  the  discharge  is 
usually  insufficient  to  move  the  heavier  material  along  the  stream 
bed  and  only  a  small  amount  of  fine  material  capable  of  being 
carried  in  suspension  flows  into  the  stream.  As  the  discharge  in- 
creases, however,  the  capacity  of  a  stream  to  transport  solid 
material  rapidly  increases,  though  the  amount  and  character  of 
the  material  transported  is  more  dependent  on  the  character  of  the 
soil  on  which  the  rain  falls  than  on  the  capacity  of  the  river  channel 
to  carry  it  away.  The  surface  flow  from  a  prairie  contains  a 
large  amount  of  fine  sand  and  clay  which  is  readily  carried  in 
suspension,  while  the  detritus  from  a  rocky  hillside  consists 
principally  of  gravel  and  coarse  sand  which  is  rolled  along  the 
river  bed. 


10  RIVERS   AND   HARBORS 

Material  that  is  in  suspension  while  moving  with  the  velocity 
of  the  water  is  also  deposited  when  this  velocity  is  reduced  for  any 
cause.  As  a  stream's  discharge  increases,  low  lands  are  gradually 
submerged,  and  since  the  velocity  of  the  overflow  is  much  less 
than  that  of  the  main  stream,  there  is  a  deposit  of  sediment, 
which  is  greatest  where  the  change  in  velocity  first  occurs,  and 
gradually  diminishes  as  the  amount  of  material  in  the  water  is 
reduced  by  the  deposition.  By  this  means  a  silt-bearing  stream 
builds  up  its  bank,  producing  the  characteristic  of  alluvial  valleys 
that  the  ground  close  to  the  river  channel  during  medium  stages 
of  the  water  is  higher  than  the  land  more  distant.  The  rainfall 
in  such  valleys,  instead  of  flowing  directly  into  the  river,  flows 
away  from  it  and  toward  the  area  of  deposition  from  the  bordering 
hills,  where  (augmented  by  the  discharge  from  the  hills)  it  creates 
a  stream  which  gradually  attains  sufficient  volume  to  erode  a 
channel  through  the  ridge  which  forms  the  banks  of  the  main 
river  and  thereupon  discharges  into  the  river. 

As  these  silt  deposits  have  been  made  during  geological  eons  of 
time,  they  frequently  attain  great  depth,  and  the  main  river  which 
has  been  depositing  sediment  in  its  bed  as  well  as  on  its  banks  may 
be  flowing  on  a  ridge,  its  thalweg  having  a  higher  elevation  than 
the  surface  of  the  ground  at  the  foot  of  the  hills  that  limit  its 
valley.  The  alluvium  thus  formed  is  readily  eroded  whenever 
it  is  attacked  by  water  flowing  with  a  greater  velocity  than  that 
which  deposited  it,  and  a  change  in  the  direction  of  the  current 
even  at  medium  or  low  stages  may  cause  the  banks  to  cave.  For 
this  reason  also  the  closure  of  a  crevasse  in  a  levee  line  on  such 
alluvial  soils  as  are  found  in  the  Mississippi  Valley  becomes  a 
difficult  problem,  if  the  water  is  flowing  through  it  with  a  depth 
exceeding  six  feet. 

When  the  precipitation  takes  the  form  of  snow,  it  neither  enters 
the  soil  nor  flows  off  its  surface  immediately.  The  density  of  a 
snow  covering  varies  from  that  of  ice,  when  it  falls  as  sleet  or 
hail,  to  a  light  fluffy  substance  which  is  exceedingly  porous  when 
first  deposited  but  which  is  liable  to  be  drifted  by  winds  into 
large  snow  banks  where  it  becomes  more  compacted.  A  light 
rain  followed  by  freezing  weather  may  convert  the  surface  of 
porous  snow  into  a  crust  of  impervious  ice. 

When  snow  is  exposed  to  the  melting  effect  of  the  sun's  rays,  its 
transformation  into  water  is  gradual,  on  account  of  its  latent 


THE   FORMATION   OF  RIVERS  11 

heat,  and  its  run-off  resembles  that  from  a  spring;  but  if  exposed 
to  a  warm  rain  or  warm  winds,  unless  a  crust  of  ice  exists  which 
will  protect  it  from  percolation,  the  snow  becomes  saturated  with 
water,  which,  absorbing  the  latent  heat,  suddenly  melts  it.  The 
resulting  flow  is  added  to  that  of  the  delayed  precipitation,  and 
the  entire  mass  starts  with  destructive  violence  on  its  path  to  the 
sea.  A  layer  of  ice  also  prevents  soil  percolation,  and  thus  in- 
creases surface  flow.  The  combination  of  a  layer  of  sleet  formed 
in  early  winter,  making  an  impervious  crust  on  a  hillside,  and 
covered  later  with  a  thick  coating  of  porous  snow  melted  in  the 
spring  by  a  heavy  fall  of  rain,  will  produce  a  most  destructive  flood. 
Freezing  weather  largely  reduces  surface  flow  by  converting  the 
water  into  ice,  but  even  with  the  surface  flow  thus  diminished, 
high  stages  may  occur  in  a  stream  on  account  of  the  formation  of 
ice  gorges. 

The  precipitation  at  any  locality  is  very  variable,  not  only  in 
the  amount  which  falls  per  day,  per  month,  and  per  year,  but  even 
in  averages  of  ten-year  intervals.  Thus  in  the  records  of  precipi- 
tation at  New  York  City,  a  period  of  ten  years  can  be  selected 
in  which  the  average  rainfall  exceeds  forty-eight  inches  per 
annum,  and  another  in  which  it  is  only  thirty-five  inches.  There 
is  also  a  great  variation  in  the  amount  and  intensity  of  the  rainfall 
at  different  localities  in  the  same  basin.  Even  within  the  limits 
of  a  city  there  may  be  a  marked  difference  in  the  discharge  of 
sewers  during  the  same  storm,  due  to  this  cause. 

The  discharge  of  a  river  which  drains  a  large  area  is  affected 
not  only  by  the  variation  in  the  amount  of  precipitation  but  also 
by  the  difference  in  the  time  required  by  both  the  subterranean 
and  the  surface  flows  to  empty  into  it.  In  a  stream  that  derives  its 
waters  from  a  single  hill,  extreme  variations  in  high  and  low  water 
are  frequent;  but  as  the  drainage  area  increases,  the  probability 
of  the  superposition  of  either  the  extreme  high  or  low  stages  of 
the  various  tributaries  which  form  the  main  stream  diminishes, 
since  the  shorter  tributaries  or  those  with  the  steeper  slopes 
deliver  their  maximum  or  minimum  discharge  earlier  than  those 
of  greater  length  or  those  of  gentler  slope.  The  subterranean 
flow  is  similarly  retarded. 

The  reservoir  capacity  of  the  river  bed  also  tends  to  diminish 
the  maximum  discharge  and  to  increase  the  minimum  discharge, 
because  a  large  amount  of  water  is  expended  in  filling  the  bed  as 


12  RIVERS  AND   HARBORS 

the  river  rises,  and  runs  out  as  it  falls,  the  duration  of  the  stage 
being  thereby  increased,  and  the  maximum  or  minimum  discharge 
correspondingly  modified.  Both  high  and  low  stages  may  be  so 
prolonged,  however,  that  the  reservoir  capacity  of  the  channel  is 
exhausted,  but  the  longer  the  river  and  the  greater  its  drainage 
area,  the  less  frequently  either  extreme  high  or  low  water  occurs. 
As  a  result,  a  river,  even  at  low  stages,  flows  on  its  lower  reaches 
with  sufficient  velocity  to  cave  its  banks  and  create  a  channel 
proportionate  to  its  volume  in  the  fine  sediment  which  has  been 
deposited  there. 

The  degradation  of  the  hills  and  the  filling  of  the  valleys  has 
continued  for  geological  ages,  gradually  raising  the  surface  of  the 
ground  in  the  valley  of  a  river  and  increasing  its  length  by  deposits 
at  its  mouth.  During  the  period,  the  water  which  has  been 
transporting  this  material  has  been  creating  and  maintaining  a 
channel  through  the  deposits,  so  that  at  the  present  time  it  has 
been  found  by  observation  that,  while  the  beds  of  streams  near 
their  head  waters  are  slowly  rising,  an  unstable  equilibrium  exists 
between  the  forces  which  are  filling  and  those  which  are  excavating 
the  channel,  at  a  relatively  short  distance  from  their  sources. 
At  certain  stages  at  a  given  locality  the  river  bed  may  be  enlarged 
and  at  other  stages  a  fill  may  occur.  If  high  stages  predominate, 
for  several  years,  the  capacity  of  the  high-water  channel  may  be 
increased,  and  during  a  series  of  low-water  seasons  the  capacity 
may  be  diminished.  But  when  former  conditions  recur,  while 
there  may  be  numerous  local  changes,  the  same  channel  capacity 
of  the  river  as  a  whole  will  be  reestablished,  unless  the  forces  of 
nature  have  been  modified  in  the  interim  by  the  works  of  man. 
Under  these  conditions,  the  average  discharge  of  solid  matter 
over  the  banks  of  a  river  and  at  its  mouth  equals  the  average 
amount  it  receives. 

The  areas  of  deposition  at  the  head  waters  of  a  stream  have  an 
effect  on  the  supply  of  solid  material  to  a  river,  similar  to  the 
effect  of  a  lake  on  the  supply  of  its  fluid  contents.  During  a 
storm  they  retain  a  large  amount  of  the  heavier  debris,  gradually 
delivering  it  to  the  river  during  minor  floods.  They  thus  act  like 
huge  rock  crushers,  grinding  the  larger  fragments  which  pass 
through  them  into  small  particles.  When  they  emerge  from  the 
area  of  deposition  these  particles  are  readily  moved  by  the  current 
which  exists  on  the  lower  portions  of  the  stream. 


THE   FORMATION   OF  RIVERS  13 

While  the  valleys  of  most  of  the  rivers  of  the  United  States  have 
been  formed  as  explained  above,  there  are  some  which  have  been 
created  by  glacial  action.  Moving  ice  also  has  a  powerful  erosive 
effect,  and  the  eroded  material,  when  once  imbedded  in  a  glacier, 
may  be  carried  long  distances  before  being  deposited.  Its  area  of 
deposition  is  the  moraine  at  the  foot  of  the  glacier  where  the  ice 
is  being  melted  by  the  sun's  rays,  and  where  there  is  formed  a 
heterogeneous  pile  of  boulders,  stone,  sand,  gravel,  and  clay,  far 
different  from  the  graded  material  found  in  alluvial  valleys. 

As  the  glacier  recedes,  the  eroded  valley  is  paved  with  a  similar 
heterogeneous  mixture.  The  glacier  also  excavates  deep  holes  in 
its  valley  which  later  become  lakes.  When  the  glacier  finally 
disappears,  and  the  erosion  of  the  hills  is  resumed  by  water  derived 
from  precipitation,  a  different  condition  exists  than  in  those 
valleys  which  owe  their  formation  through  geological  ages  to 
such  precipitation  alone.  At  the  foot  of  the  hills  limiting  the 
valley,  areas  of  deposition  are  formed  which  gradually  encroach 
on  the  valley,  but  the  water  escaping  from  these  areas  flows  into 
the  existing  lakes  and  there  deposits  its  solid  matter.  Emerging 
from  the  lakes  it  flows  as  a  clear  water  stream  of  diminished 
velocity  and  passes  over  the  surface  of  ground  that  had  been 
eroded  by  a  force  far  greater  than  it  possesses.  Its  sinuosities 
and  depth  become  functions,  not  of  its  discharge,  but  of  the 
character  of  the  deposits  left  by  the  glacier.  Instead  of  having 
steep  slopes  at  its  source  gradually  diminishing  toward  its  mouth, 
it  is  liable  to  flow  with  gentle  slopes  in  its  upper  reaches  until  it 
comes  to  obstructions  deposited  by  the  glacier,  over  which  it 
flows  in  rapids,  not  having  sufficient  force  to  excavate  its  channel 
through  them.  As  such  a  stream  carries  little  sediment,  it  is  in- 
capable of  building  up  its  banks  and  the  rain  which  falls  in  its 
valley,  instead  of  flowing  from  the  river  to  the  foothills,  as  in 
alluvial  streams,  has  a  reverse  flow. 

In  such  valleys,  the  lakes  are  gradually  filling  and  are  being 
converted  into  marshes,  and  in  future  geological  ages  they  will 
be  transformed  into  valleys  similar  to  those  first  described.  In 
certain  valleys,  the  transformation  is  more  complete  than  in 
others,  and  there  is  found  a  foundation  of  glacial  drift  covered  by 
a  relatively  thin  layer  of  alluvial  deposits. 

A  glacier  has  a  powerful  erosive  effect  on  rock,  but  is  very 
eccentric  in  its  action,  occasionally  leaving  rock  ridges  extending 


14  RIVERS   AND   HARBORS 

across  its  valley  which  the  clear  water  of  the  river  which  forms  on 
the  glacier's  disappearance  is  unable  to  remove.  In  the  upper 
valleys  of  streams  formed  by  water  derived  from  precipitation,  deep 
canyons  may  be  excavated  through  rock  by  the  erosion  of  the  de- 
bris which  is  being  carried  down  them. 

In  the  area  of  deposition  of  an  alluvial  valley,  as  in  one  formed 
by  glacial  action,  the  channel  formed  by  a  stream  is  dependent  on 
the  eccentricities  of  the  deposition  of  debris.  A  flood  carries  the 
sand,  gravel,  and  boulders  down  the  ravines  and  spreads  them  in 
a  fan-shaped  mass  across  their  entrances.  As  the  flood  subsides, 
pools  will  be  scoured  out  where  the  deposit  is  of  sand  and  gravel, 
and  between  the  pools  the  water  will  flow  in  shallow  ripples  over 
material  it  is  incapable  of  moving.  Such  a  channel  is  not  per- 
manent but  is  liable  to  be  obliterated  by  the  deposits  from  the 
next  flood,  and  a  new  channel  may  be  formed  where,  due  to  some 
freak  of  nature,  material  more  readily  moved  is  found. 

But  as  the  detritus  becomes  broken  up  into  smaller  particles  in 
its  passage  through  the  area  of  deposition,  it  becomes  more  amen- 
able to  the  action  of  the  stream  currents,  and  the  channel  assumes 
a  more  stable  form.  There  is  still  a  large  amount  of  material  being 
carried  down  the  river  during  floods,  but  after  each  flood  there  is 
a  greater  tendency  for  the  stream  to  return  to  the  channel  it 
formerly  occupied  at  similar  stages.  The  pools  develop  in  the 
bends  of  the  stream,  and  are  separated  by  bars  of  heavy  material. 
In  other  words,  the  sediment  flows  in  accordance  with  certain 
laws,  and  it  is  as  important  to  comprehend  clearly  these  laws  as  it 
is  to  comprehend  those  of  the  flow  of  water  (1). 


CHAPTER  III 
LAWS   GOVERNING  THE  FLOW  OF  WATER  IN  RIVERS 

The  Chezy  formula *  and  those  derived  from  it,  such  as  Bazine's, 
Farming's,  and  Kutter's,  are  the  result  of  careful  experiments 
from  which  numerical  values  have  been  deduced  for  empirical 
coefficients  which  render  the  formulas  of  great  assistance  in  solv- 
ing hydraulic  problems,  but  they  have  the  defect  of  all  empirical 
formulas  that  great  caution  must  be  exercised  in  applying  them 
where  conditions  differ  from  those  of  the  experiments. 

These  formulas  as  a  class  were  first  derived  from  the  flow  of 
water  in  pipes  and  conduits,  and  it  was  attempted  to  concentrate 
in  a  coefficient  c  all  the_variations  of  the  flow  which  prevented  v 
from  being  equal  to  Vrs.  It  was  soon  observed,  however,  that 
c  was  a  variable  function  of  both  r  and  s,  and  tables  were  prepared 
giving  values  of  the  coefficient  for  changes  in  both  these  quan- 
tities. It  was  also  found  by  experiment  that  the  condition  of  the 
enveloping  surface  affected  the  velocity  of  the  discharge,  a  smooth 
iron  or  vitrified  clay  conduit  discharging  more  water  in  a  given 
time  than  a  brick  one  of  the  same  diameter  and  laid  to  the  same 
slope.  Changes  in  the  value  of  the  coefficient  then  became 
necessary  to  provide  for  the  change  in  velocity  due  to  the  rough- 
ness of  the  surface  of  the  material  composing  the  conduit. 

The  frictional  resistance  of  a  liquid  flowing  over  a  solid  sub- 
stance is  slight  and  affects  only  the  thin  layer  that  is  in  contact 
with  the  solid.  In  a  large  conduit,  such  resistance  is  negligible 
and  the  retarding  effect  of  a  rough  surface  on  the  flow  is  due,  not 
to  such  frictional  resistance,  but  to  the  eddies  which  it  creates 
in  the  liquid  itself  and  which  absorb  a  portion  of  the  energy  that 
would  otherwise  be  expended  in  producing  a  velocity  of  discharge 
through  the  conduit.  Hence  the  term  coefficient  of  roughness  does 
not  give  a  proper  conception  of  this  force.  The  velocity  may  be 
such  as  to  develop  frictional  resistance  only. 

There  exist  two  critical  stages,  as  they  have  been  termed,  a 
minimum  stage  when  the  velocity  is  so  low  that  eddy  action  is 

1  v  =  cVrs  in  which  »  =  the  velocity;  c  =  a  variable  coefficient;  r  =  the  hydraulic  radius; 
and  s  =  the  slope. 

15 


16  RIVERS  AND   HARBORS 

not  developed,  and  a  maximum  stage  in  which  mere  occurs  the 
greatest  retarding  eddy  action  the  velocity  can  create  (1).  This 
maximum  stage  is  dependent  on  the  form  assumed  by  the  particles 
producing  the  roughness.  Both  below  the  minimum  critical 
stage  and  above  the  maximum  critical  stage  there  is  a  tendency 
for  the  flow  to  follow  rectilinear  lines  instead  of  producing  eddies. 
As  an  illustration,  suppose  that  a  straight  channel  be  excavated 
through  rock  and  that  the  sides  and  bottom  be  left  as  blasted. 
If  the  slope  is  gentle,  a  small  discharge  will  flow  through  the  cut 
without  a  serious  disturbance  of  the  water  surface.  As  the  dis- 
charge and  its  velocity  increase,  boils  and  eddies  begin  to  appear, 
which  attain  a  maximum  at  a  certain  stage.  Above  this  stage 
the  eddies  gradually  disappear  on  the  surface,  having  been  limited 
to  certain  distances  from  the  bottom  and  sides  of  the  cut,  beyond 
which  the  current  tends  to  flow  in  straight  lines.  On  the  con- 
trary, if  the  same  discharge  flows  in  a  channel  of  sand  which  has 
been  deposited  in  sand  waves,  the  minimum  critical  stage  is  not 
attained  so  soon,  i.e.,  the  water  flows  tranquilly  at  low  stages, 
but  at  the  velocity  of  a  maximum  discharge,  the  water  moving  up 
the  surface  of  the  ridges  continues  on  the  same  path  to  the  water 
surface  and  boils  and  eddies  of  great  magnitude  result. 

Furthermore  the  length  and  height  of  the  sand  wave  affect  its 
capacity  to  reduce  velocity.  Sand  waves  deposited  during  low 
water,  though  composed  of  the  same  material  and  more  frequent  in 
number,  have  a  lower  coefficient  of  roughness  during  floods  than 
those  deposited  during  high  stages. 

As  the  experiments  from  which  the  ordinary  formulas  were  de- 
rived were  for  ordinary  flow  between  the  critical  stages  described 
above,  their  application  to  either  extremely  low  or  extremely 
high  velocities  should  be  made  with  caution.  The  subterranean 
flow  of  water  is  so  slow  that  it  is  below  the  minimum  critical 
stage,  and  the  velocity  has  been  found  to  vary  almost  directly 
as  the  head.  The  same  rule  applies  to  the  discharge  of  small 
pipes.  With  brass  pipes  of  a  diameter  as  great  as  two  inches 
under  low  heads  a  formula  v1M—crs  has  been  found  to  be  appli- 
cable. Hazen  and  Williams  have  proposed  a  formula  with  a 
variable  exponent  for  v  as  a  substitute  for  the  ones  generally 
employed,  and  this  is  theoretically  more  exact. 

When  it  is  attempted  to  apply  these  formulas  to  river  channels, 
as  in  the  experiments  of  Kutter,  serious  difficulties  arise.  The 


FLOW   OF  WATER   IN   RIVERS  17 

formulas  were  derived  from  uniform  flow  in  conduits  or  pipes 
which  had  the  same  hydraulic  radius  in  different  sections,  and 
uniform  slope.  Such  conditions  do  not  exist  in  rivers,  for  no 
two  cross-sections  of  a  river  have  the  same  area,  and  the  area  of 
the  same  cross-section  changes  from  day  to  day.  Moreover,  the 
slopes  are  exceedingly  variable,  being  not  only  steeper  over  bars 
than  in  pools,  but  frequently  varying  on  opposite  sides  of  the 
same  pool.  Finally,  the  coefficient  of  roughness  varies  from 
section  to  section,  being  greater  on  a  gravel  bar  than  in  a  sandy 
pool. 

In  applying  a  formula  it  is  therefore  necessary  to  consider  long 
reaches  of  the  river,  obtaining  a  mean  hydraulic  radius  and  a 
mean  slope.  If  it  is  attempted  to  apply  the  formula  to  a  short 
stretch  which  happens  to  have  a  uniform  section  and  slope,  the 
velocity  of  approach  becomes  a  disturbing  factor,  the  propelling 
force  being  not  the  difference  in  head  at  .the  two  extremities  of 
the  section,  but  that  created  by  slopes  further  upstream.  Even  a 
strong  wind  influences  the  discharge  of  a  river,  since  it  raises  or 
lowers  the  thread  of  maximum  velocity.  At  the  mouths  of 
rivers,  tides  may  have  sufficient  force  (on  account  of  the  funnel- 
shaped  form  of  the  estuary)  to  cause  a  flow  in  a  reverse  direction 
to  the  low-water  slope;  when  the  tides  introduce  salt  water  into 
the  channel,  they  produce  a  most  complicated  flow,  incapable  of 
mathematical  analysis. 

It  is  therefore  advisable  when  discussing  questions  of  river 
hydraulics  to  determine  by  actual  measurements  the  various  ele- 
ments entering  a  formula.  When  this  is  impracticable,  the  reader 
is  cautioned  to  employ  a  liberal  coefficient  of  safety,  bearing  in 
mind  that  most  mathematical  computations  are  applicable  to 
average  conditions,  and  that  in  treating  such  questions  as  floods, 
and  depths  of  channel  during  extreme  low  water,  it  is  not  the 
average  but  the  exceptional  that  has  to  be  considered. 

In  a  pipe  the  velocity  of  flow  is  determined  by  the  head.  In  a 
river  the  living  force  of  the  water  also  must  be  considered.  When 
the  flow  of  water  in  a  pipe  is  suddenly  checked,  the  pressure  on 
its  surface  is  merely  increased,  as  the  water  has  no  means  of 
escape.  But  in  a  river,  when  the  flow  is  restricted  at  any  locality, 
the  force  of  the  approaching  water  causes  an  elevation  of  the  con- 
tracted section  corresponding  to  the  pressure  existing  in  the  pipe. 
A  rapid  current  impinging  perpendicularly  on  a  river  bank  may 


18  RIVERS   AND   HARBORS 

reverse  the  river  slopes.  This  was  forcibly  illustrated  in  the 
Mississippi  River  at  Vicksburg  after  the  Centennial  Cut-off  of 
1876.  The  old  river  bed  around  the  island  created  by  the  cut-off 
then  became  the  harbor  of  Vicksburg  and  was  connected  to  the 
river  during  certain  stages  at  both  its  upper  and  lower  ends. 
The  flow  through  the  cut-off  during  high  stages  impinged  on  the 
Vicksburg  bank  with  sufficient  force  to  cause  such  a  local  elevation 
of  the  water  surface  as  to  create  an  upstream  current  through 
Vicksburg  harbor  until  the  river  fell  to  about  mid-stage,  and 
caused  a  heavy  deposit  of  sediment,  necessitating  the  closing  of 
the  upper  entrance  to  the  harbor  and  the  diversion  of  the  Yazoo 
river  into  it  to  restore  and  to  maintain  its  navigability. 

At  the  outlets  of  rivers  the  tidal  wave  frequently  has  a  force  far 
exceeding  that  produced  by  the  natural  slope  of  the  river  and 
causes  a  rising  of  the  river's  waters  in  excess  of  that  existing  in 
the  ocean.  The  impetus  of  the  outflowing  tides  may  also  lower 
the  river  level  below  that  of  low  water  in  the  ocean. 

The  flow  of  water  in  bends  has  such  a  vital  effect  on  a  river's 
regimen  that  it  merits  a  more  careful  analysis  than  it  ordinarily 
receives  in  textbooks  on  hydraulics.  While  there  have  been 
numerous  experiments  upon  the  retardation  of  flow  caused  by 
bends  in  pipe,  the  subject  usually  is  dismissed  with  the  statement 
that  the  loss  of  head  in  bends  is  equivalent  to  adding  a  certain 
amount  to  the  length  (2) .  It  is  more  fully  discussed  in  an  article 
by  M.  R.  H.  Gockinga  on  La  Pente  Transversale  et  son  Influence 
sur  VEtat  des  Rivieres  (ANNALES  DBS  FONTS  ET  CHAUSS^ES,  1913, 
p.  112). 

In  a  straight  paved  conduit  the  filaments  of  water  flow  in  straight 
lines,  with  the  exception  of  those  near  its  bed,  which  are  disturbed 
by  the  roughness  of  the  material  with  which  they  come  in  contact. 
The  thread  of  maximum  velocity  lies  in  the  vertical  plane  passing 
through  the  deepest  section,  and  the  variations  of  velocity  of  the 
filaments  of  water  in  that  plane  will  conform  to  the  arc  of  a  parab- 
ola whose  apex  is  near  the  water  surface.  A  similar  curve  will 
determine  the  variations  in  velocities  from  the  plane  of  maxi- 
mum velocities  to  the  sides  of  the  conduit.  In  a  cross-section 
the  water  surface  will  be  horizontal.  If  the  flow  is  uniform,  the 
longitudinal  surface  slope  will  conform  to  that  of  the  conduit. 

If  a  circular  curve  be  introduced  into  the  conduit,  however, 
there  is  a  derangement  of  this  regularity  of  flow.  The  inertia  of 


FLOW   OF   WATER   IN   RIVERS  19 

the  water  resists  the  change  of  direction,  and  there  is  an  elevation 
of  the  water  surface  on  the  concave  side  of  the  conduit  and  a 
corresponding  lowering  on  the  convex  side.  The  mid-stream 
longitudinal  slope  remains  the  same,  but  there  has  been  created  a 
transverse  slope  in  the  bend  which  causes  a  marked  difference  of 
head  on  the  opposite  sides.  The  cross-section  of  the  water  surface 
becomes  a  curved  line.  For  the  case  in  which  the  cross-section 
of  the  conduit  is  trapezoidal,  Mr.  Gockinga  deduced  the  equation 


where  V  denotes  the  longitudinal  velocity,  R  denotes  the  radius  of 
curvature  of  the  axis  of  the  conduit  in  meters,  and  x  and  y  are 
coordinates  of  a  point  on  the  surface  referred  to  a  system  of  rec- 
tangular axes  through  the  intersection  of  the  axes  of  the  conduit 
and  the  water  surface.  For  a  trapezoidal  conduit  whose  width 
is  200  meters  and  whose  radius  is  500  meters,  and  whose  depth 
is  such  that  a  longitudinal  slope  of  0.0001  produces  a  velocity  of 
one  meter  per  second,  he  computes  a  difference  of  head  of  4 
centimeters  on  opposite  sides  of  the  conduit,  i.e.  a  transverse 
slope  twice  that  of  the  longitudinal. 

These  theoretical  computations  have  been  confirmed  by  obser- 
vations on  the  Mississippi  River,  where  differences  of  head  of  about 
1  foot  have  been  observed  on  opposite  banks  of  bends,  with  mean 
mid-section  longitudinal  slopes  of  about  0.4  foot  per  mile.  Under 
such  conditions  the  thread  of  maximum  velocity  no  longer  cor- 
responds with  the  mid-stream  section,  but  approaches  the  concave 
bank. 

When  two  bodies  of  water  whose  surfaces  have  different  ele- 
vations, are  connected  by  a  pipe,  there  is  a  flow  through  the  pipe 
from  the  one  having  the  greater  head,  which  will  continue  until 
the  heads  are  equalized.  The  velocity  of  the  flow  through  the 
pipe  is  a  function  of  the  difference  of  head.  If  a  variation  in  head 
exists  in  different  parts  of  a  lake,  which  usually  results  from  a 
wind  blowing  over  its  surface,  the  same  tendency  to  restore  the 
normal  levels  is  created,  but  since  the  force  of  the  wind  is  com- 
pelling the  surface  water  to  travel  in  the  same  direction  as  the  wind, 
the  return  flow  is  along  the  lake  bed.  This  rises  to  the  surface 
where  the  lake  level  is  lowest,  thereby  giving  the  water  a  curvi- 
linear path  (3)  . 

Under  certain  conditions  this  subsurface  flow  may  attain  great 


20  RIVERS   AND   HARBORS 

force.  The  vertical  piers  constructed  on  the  Great  Lakes  have  to 
be  protected  by  heavy  rip-rap  in  depths  of  thirty  feet  to  prevent 
scour  from  this  cause. 

In  a  river,  the  centrifugal  force  created  by  bends  produces  a 
similar  action,  but  the  curvilinear  path  of  the  water  in  a  lake  is 
modified  in  a  river  by  the  motion  of  the  water  downstream,  due 
to  the  longitudinal  slope.  Hence  the  water  in  a  river  assumes  a 
helicoidal  motion.  The  motion  of  translation  downstream  is  re- 
tarded, but  in  the  circumference  of  the  helicoid  the  filaments  of 
water  have  acquired  a  velocity  much  greater  than  that  which 
exists  in  a  straight  reach  due  to  the  longitudinal  slope  alone. 

The  dimensions  of  the  helicoid  are  functions  of  the  longitudinal 
slope  and  the  radius  of  curvature  of  the  bend.  They  conform  to 
the  section  in  which  the  water  flows  only  when  it  is  permitted  to 
construct  its  own  path,  i.e.,  in  a  stream  in  its  natural  state.  Beyond 
the  sphere  of  its  flow,  eddies  are  produced  which  also  interfere 
with  the  motion  downstream. 

The  helicoidal  flow  of  water  in  bends  was  practically  demon- 
strated by  Professor  James  Thompson  before  the  Institution  of 
Mechanical  Engineers  of  Glasgow  in  1879,  and  the  direction  and 
velocity  of  such  currents  on  the  Dnieper  River  were  measured  by 
M.  Leliavski,  as  described  in  a  paper  presented  to  the  Sixth  In- 
ternational Navigation  Congress  held  at  the  Hague  in  1894. 
From  the  results  of  his  experiments,  Leliavski  came  to  the  con- 
clusion that  in  a  bend,  surface  currents  converge  toward  the 
concave  bank,  along  which  a  stream  of  water  flows  to  the  bottom 
of  the  river,  thence  move  to  the  convex  side  in  divergent  currents, 
and  then  gradually  rise  to  the  surface.  He  found  not  only  that 
these  currents  have  sufficient  force  to  cause  caving  of  banks  of 
clay,  sand,  or  gravel,  but  he  found  evidences  also  that  they  had 
caused  erosion  in  the  lava  rock  which  forms  the  bed  of  the  river 
at  the  Dnieper  rapids.  The  greatest  depth  was  found  where  the 
greatest  number  of  surface  filaments  of  water  converge  on  the 
concave  bank.  If  the  radius  of  curvature  of  the  bend  is  uniform, 
this  point  occurs  at  some  distance  below  the  middle  of  the  bend. 

The  influence  of  a  circular  bend,  moreover,  affects  the  straight 
sections  of  a  conduit  for  a  considerable  distance  above  and  below 
it.  Below  the  bend,  it  is  evident  that  a  considerable  distance  is 
required  to  transform  the  helicoidal  motion  again  into  a  recti- 
linear motion,  since  this  transformation  depends  on  the  inertia 


FLOW   OF  WATER   IN   RIVERS  21 

and  the  frictional  resistance  of  the  particles  of  water.  Above  the 
bend  there  is  a  similar  back-water  effect,  since  the  surface  of  the 
cross-section  cannot  suddenly  change  from  a  horizontal  to  an  in- 
clined line,  i.e.  the  transition  must  be  gradual.  The  helicoidal 
motion  of  the  water  therefore  begins  considerably  above  the  bend, 
attains  its  maximum  velocity  when  the  transverse  slope  adjusts 
itself  to  the  radius  of  curvature,  then  remains  constant  to  the  end 
of  the  curve,  and  then  gradually  diminishes.  The  locus  of  the 
maximum  velocity  moves  away  from  the  axis  of  the  conduit  in 
a  corresponding  manner  and  there  is  a  disturbance  in  the  longi- 
tudinal velocity  of  flow  in  the  straight  sections  as  well  as  in  the 
curve. 

In  a  paved  conduit  the  location  of  the  thread  of  maximum 
velocity  is  of  minor  importance,  but  as  this  line  is  also  the  one  of 
greatest  scour,  it  determines  the  navigable  channel,  and  river 
regulation  consists  principally  in  its  proper  location  and  main- 
tenance. 

Another  question  which  requires  more  elucidation  than  is  con- 
tained in  the  ordinary  textbooks  is  the  effect  of  obstacles  on  the 
flow  of  water.  It  is  recognized  that  the  sudden  expansion  and 
contraction  of  a  conduit  entails  a  loss  of  head,  due  to  the  eddies 
formed,  but  the  paths  followed  by  the  water  in  passing  the  ob- 
stacles have  received  little  consideration. 

In  a  straight  conduit  not  susceptible  to  erosion,  if  a  vertical  dike 
extending  above  the  water  surface  be  constructed  perpendicular 
to  one  of  its  sides,  the  reduction  of  the  area  of  cross-section  will 
cause  a  local  elevation  of  the  water  surface,  reducing  the  slope 
above  the  obstacle  and  increasing  it  below.  This  elevation, 
while  extending  across  the  entire  cross-section,  will  be  greatest 
near  the  side  of  the  conduit  above  the  dike,  due  to  the  greater 
retardation  of  the  longitudinal  velocity  at  that  locality,  and  the 
least  on  the  opposite  side.  This  difference  of  head  will  induce  a 
flow  toward  the  points  of  lower  elevation  and  the  locus  of  the 
thread  of  maximum  velocity  will  be  diverted  from  the  straight 
line  it  followed  when  the  conduit  was  unobstructed.  For  a 
certain  distance  above  the  dike,  back-water  effects  will  produce 
a  difference  of  elevation  on  opposite  sides  of  the  conduit,  and  a 
helicoidal  motion  will  be  imparted  to  the  water  similar  to  that 
in  bends.  Below  the  dike  conditions  are  suddenly  reversed.  The 
water  behind  the  dike  has  lost  its  longitudinal  velocity  and  tends 


22  RIVERS   AND   HARBORS 

to  sink  to  a  lower  level  than  formerly  existed,  while  on  the  opposite 
side  the  elevation  has  increased.  A  portion  of  the  water  flowing 
around  the  obstacle  seeks  to  fill  this  void,  but  because  it  has  a 
high  longitudinal  velocity  as  it  passes  the  dike,  it  forms  an  eddy 
of  a  very  complicated  flow  but  with  a  large  spiral  component 
with  a  vertical  axis.  In  experiments  on  the  Rhine  it  (4)  was  found 
that  this  spiral  eddy  attacked  the  bank  at  a  distance  below  the 
dikes  equal  to  about  two  and  one-half  times  its  length.  Another 
component  has  a  quasi-helicoidal  flow  diagonally  toward  the  op- 
posite side,  which  also  converts  some  of  the  longitudinal  velocity 
into  eddy  currents. 

If  a  similar  dike  is  constructed  on  the  opposite  side  of  the 
conduit  in  the  same  cross-section,  the  water  surface  between  the 
dikes  assumes  a  curved  form,  higher  at  the  extremities  of  the  dikes 
than  in  mid-stream.  The  locus  of  maximum  velocities  divides  into 
two  lines  diverging  upstream  from  the  axis  of  the  conduit  to  the 
ends  of  the  dikes,  and  gradually  returning  to  the  middle  section 
below  them.  Two  powerful  eddies  are  produced  behind  the 
dikes,  but  the  remainder  of  the  flow  tends  to  follow  lines  more 
rectilinear  than  those  which  exist  when  only  one  obstruction  is 
formed. 

There  is  also  eddy  action  above  a  dike,  but  its  nature  and  extent 
is  a  function  of  the  inclination  of  the  dike  to  the  direction  of  the 
current.  If  the  dike  is  inclined  sufficiently  downstream,  it  has 
an  action  similar  to  that  of  a  curve  and  a  tendency  to  scour  de- 
velops along  its  face.  This  tendency  is  diminished  as  the  dike  is 
given  a  greater  inclination  upstream. 

If  the  dike  be  submerged  there  is  a  tendency  toward  the  crea- 
tion of  a  jump  in  the  water  surface  over  it.  A  very  complicated 
flow  results.  The  overflow  produces  a  powerful  scouring  effect 
immediately  below  the  dike,  and  interferes  with  the  eddy  action 
around  its  end. 

On  a  concave  bank,  a  dike  causes  a  curvature  of  the  line  of  maxi- 
mum velocity,  but  it  cannot  ordinarily  overcome  the  centrifugal 
force  created  by  the  bend  and  the  line  soon  returns  to  its  normal 
position. 

The  discharge  of  a  tributary  does  not  immediately  mingle  with 
the  water  of  the  main  stream,  but  flows  beside  it  until  bends  and 
obstacles  (by  their  helicoidal  and  eddy  action)  interfere  with  the 
regularity  of  flow. 


FLOW    OF   WATER   IN   RIVERS  23 

Near  the  mouths  of  rivers,  a  very  complicated  flow  frequently 
results  from  the  difference  in  density  of  fresh  and  salt  water. 
The  water  of  the  inflowing  tide  has  a  greater  specific  gravity  than 
that  of  the  river  discharge.  At  certain  periods  of  the  tide  the 
salt  water  flows  up  the  river  along  its  bed,  while  the  fresh  water 
is  flowing  out  above  it.  On  account  of  irregularities  in  the  river 
bed,  there  may  also  be  an  upstream  surface  flow  in  certain  por- 
tions of  a  river  section  with  a  downstream  flow  in  others,  until 
the  inertia  of  the  moving  water  is  overcome  and  the  salt  and 
fresh  waters  have  an  opportunity  to  mingle. 


CHAPTER  IV 
THE  FLOW  OF  SEDIMENT  IN  NON-TIDAL  RIVERS 

Material  is  transported  down  a  river  in  solution,  in  suspension, 
and  by  being  rolled  along  its  bed.  The  material  in  solution  is 
carried  to  the  mouth  of  the  river  without  deposition  unless  there 
is  excessive  evaporation.  Streams  flowing  in  valleys  formed  by 
glacial  action  not  infrequently  carry  more  material  in  solution 
than  in  suspension,  as  is  demonstrated  by  the  observations  of  the 
Geological  Survey  on  the  Mississippi  River  at  Minneapolis,  where 
the  mean  of  the  observations  shows  200  parts  per  million  in  solu- 
tion and  7.9  parts  per  million  in  suspension. 

In  a  river  flowing  in  an  alluvial  valley,  the  reverse  is  the  case 
except  occasionally  at  extreme  low  water,  and  the  amount  of  ma- 
terial carried  in  suspension  is  primarily  dependent  on  the  character 
of  the  soil  of  the  watershed  from  which  it  flows  and  on  the  intensity 
of  the  rainfall.  The  western  tributaries  of  the  Mississippi  River 
carry  a  greater  amount  of  sediment  per  unit  of  volume  of  water, 
termed  the  degree  of  saturation,  than  its  eastern  tributaries,  and 
(for  the  same  discharge)  the  degree  of  saturation  is  greater  in 
summer  than  in  the  winter,  due  to  the  frozen  condition  of  the  soil 
in  winter.  The  light  clays  and  sands  which  are  carried  in  sus- 
pension become  intimately  mixed  with  the  water  as  it  flows  over 
the  soil  and  (moving  with  the  velocity  of  the  water)  are  carried 
to  the  river's  mouth  unless  the  velocity  is  checked,  as  in  the  flow 
through  a  lake. 

In  a, watershed  whose  soil  contains  a  large  amount  of  clay  and  is 
of  a  relatively  uniform  character  over  the  drainage  area,  the  degree 
of  saturation  rapidly  increases  with  an  increase  in  the  discharge. 
In  the  Mississippi  system  of  rivers,  however,  the  Missouri  carries 
so  much  more  sediment  in  suspension  than  any  of  the  other  tribu- 
taries that  it  determines  the  degree  of  saturation  of  the  main 
stream,  and  a  flood  from  the  Missouri  River  carries  more  material 
in  suspension  to  the  Gulf  than  one  from  the  Ohio,  although  the 
amount  of  water  flowing  from  the  latter  in  floods  largely  exceeds 
that  from  the  former. 

When  material  carried  in  suspension  has  once  been  deposited, 

24 


SEDIMENT  IN   NON-TIDAL  RIVERS  25 

and  is  afterwards  eroded,  -only  a  comparatively  small  portion  is 
again  placed  in  suspension.  The  original  deposit  contains  a  large 
amount  of  water,  and  assumes  gentle  slopes,  but  it  becomes  more 
compacted  and  is  capable  of  maintaining  a  steeper  slope  when 
additional  deposition  occurs.  If  the  river  falls  to  such  a  stage  that 
the  deposit  is  above  the  water  surface,  and  the  water  is  drained 
from  it,  the  binding  force  of  its  clay  contents  may  be  sufficient  to 
enable  the  material  to  assume  a  vertical  slope,  and  thus  produce 
the  steep  banks  found  above  low  water  in  the  concave  bends  of 
alluvial  streams.  Below  the  water  surface,  the  adhesive  force  of 
the  clay  diminishes,  and  the  water  contents  of  the  deposit  increase. 
When  such  a  bank  is  eroded,  there  is  a  local  deepening  of  the 
river  bed,  and  an  increase  in  the  under-water  slopes,  which  cause 
large  masses  to  slide  into  the  river.  These  masses  do  not  mingle 
with  the  water  sufficiently  to  be  carried  in  suspension.  When  rain 
falls  on  a  plowed  field  every  drop  of  water  picks  up  a  load  of  clay 
or  sand  which  it  can  transport;  but  a  mass  of  water  which  acts 
on  a  concave  bank  moves  a  mass  of  earth  too  heavy  to  float,  and 
which  therefore  is  rolled  along  the  river  bed. 

There  is  also  a  relation  between  a  river's  low-water  slope  and 
the  amount  of  sediment  it  carries;  the  greater  the  amount  of 
sediment,  the  steeper  becomes  the  slope.  This  is  particularly 
noticeable  at  the  junction  of  two  streams.  If  a  river  carries 
relatively  little  sediment,  its  slope  above  the  junction  with  a  turbid 
stream  is  less  than  that  of  the  tributary,  and  increases  below  it. 
If  the  reverse  is  the  case,  the  slope  of  the  main  stream  above  the 
junction  is  increased,  and  below  it  more  nearly  conforms  to  that  of 
the  tributary. 

Exceptions  to  the  rule  result  from  the  geological  formation  of 
non-erosive  beds  in  the  vicinity  of  the  junction.  For  example, 
the  upper  Mississippi  carries  less  sediment  than  the  Minnesota, 
its  first  large  tributary,  but  the  falls  of  St.  Anthony  above  the  junc- 
tion and  the  rapids  produced  by  the  detritus  from  the  falls  create 
an  exceptional  condition. 

The  rule  is  illustrated  by  the  following  instances:  The  waters 
of  the  next  large  tributary,  the  St.  Croix  River,  have  been  clari- 
fied during  their  passage  through  Lake  St.  Croix,  and  the  mean 
slope  of  the  Mississippi  above  their  junction  is  about  0.4  foot  per 
mile,  below  it  0.2  foot.  At  the  junction  with  the  Chippewa, 
which  transports  a  large  amount  of  coarse  sand  and  but  little 


26  KIVERS  AND   HARBORS 

finer  material,  the  low-water  slope  through  Lake  Pepin  is  zero,  and 
immediately  below  the  mouth  of  the  Chippewa  0.9  feet  per  mile. 
At  the  mouth  of  the  Wisconsin,  which  also  carries  large  amounts  of 
sand  during  floods,  the  slope  of  the  main  river  above  it  is  0.1  foot, 
and  below  0.6  foot  per  mile.  The  western  tributaries  of  the 
river  below  have  sufficient  material  in  suspension  to  maintain  an 
average  slope  of  0.4  foot  per  mile  to  the  Illinois  River,  with  the 
exception  of  the  Rock  Island  and  DesMoines  rapids.  The  Illinois 
River  carries  little  sediment,  and  the  slope  above  its  junction  ex- 
ceeds 0.4  foot  per  mile;  below  it  a  gentler  slope  is  observed  as  far 
as  the  Missouri  River.  Below  the  Missouri  River  a  slope  exceed- 
ing 0.8  foot  per  mile  is  created,  gradually  reducing  to  0.6  foot,  and 
below  the  junction  with  the  Ohio  still  gentler  slopes  exist  for  a 
considerable  distance. 

These  phenomena  are  usually  explained  by  the  assumption  that 
they  are  caused  by  the  relative  degree  of  saturation  of  the  two 
streams,  and  that  when  their  waters  mingle  they  are  capable  of 
increasing  their  capacity  for  transporting  material  in  suspension, 
the  less  turbid  waters  increasing  their  load  from  material  eroded 
from  the  bed  of  the  river  ( 1 ) .  Observations  by  the  Mississippi  River 
Commission  fail  to  confirm  this  assumption.  At  the  junction  of 
the  Missouri  and  upper  Mississippi  it  was  noted  that  the  waters 
of  the  two  rivers  have  a  tendency  to  flow  side  by  side  without 
mingling,  the  waters  of  the  upper  Mississippi  following  the  Illinois 
bank  of  the  river  and  the  waters  of  the  Missouri  the  opposite  bank. 
At  low  stages  of  the  Missouri,  this  tendency  continues  considerably 
beyond  the  portion  of  the  river  having  a  steep  slope.  Boils  and 
eddies  cause  a  gradual  mixing  of  the  waters,  and  during  certain 
high  stages,  at  the  first  concave  bend  below  the  junction  which 
occurs  on  the  Missouri  bank,  there  is  a  decided  movement  of  the 
water  of  the  upper  Mississippi  across  the  channel  on  the  surface, 
and  an  opposite  bottom  flow  of  Missouri  River  water,  which  is 
exhibited  in  the  observations  of  sediment  taken  at  that  locality. 

A  more  logical  explanation  of  the  changes  of  slope  at  the  mouths 
of  tributaries  is  to  be  found  in  the  deposition  of  material  in  sus- 
pension, and  its  conversion  into  sand  waves  which  are  moved  along 
the  river  bed.  As  the  crests  of  floods  of  rivers  rarely  coincide, 
when  a  clear-water  stream  empties  into  a  turbid  river  there  will 
be  a  period  during  a  high  stage  of  the  tributary  when  its  discharge 
will  act  as  a  dam  on  that  of  the  main  river,  diminishing  the 


SEDIMENT   IN   NON-TIDAL   RIVERS  27 

velocity  of  flow  above  the  junction  and  causing  a  deposit  of  ma- 
terial carried  in  suspension  which  will  reduce  the  river's  cross- 
section.  Below  the  junction  its  added  waters  will  tend  to  enlarge 
the  cross-section. 

When  the  tributary  falls  to  its  normal  relation  to  the  main 
stream,  the  velocity  above  the  junction  is  increased  and  the 
deposits  tend  to  scour,  being  slowly  rolled  along  the  bottom  as 
sand  waves  instead  of  being  carried  in  suspension  with  the  velocity 
of  the  current.  There  is  a  tendency  for  this  scoured  material  to 
deposit  in  the  enlarged  section  below  the  junction,  but  before  the 
equilibrium  can  be  established  a  second  rise  occurring  in  the 
tributary  causes  a  repetition  of  the  process. 

If  the  main  stream  is  less  turbid  than  the  tributary,  a  flood  in  the 
latter  flows  into  an  enlarged  section  and  consequently  deposits 
sediment  due  to  a  reduction  in  velocity.  As  the  tributary  falls, 
the  main  stream  has  an  increased  burden  in  removing  the  deposits 
below  the  junction,  a  work  it  also  fails  to  accomplish  before  a 
second  rise  occurs  in  the  tributary. 

The  serious  problem  in  river  regulation  is  the  movement  of 
material  along  the  river  bed.  In  straight  reaches  it  moves  in 
sand  waves  which  are  functions  of  the  velocity  of  the  current  and 
of  the  depth  of  water,  being  greatest  when  for  any  cause  the 
current  is  suddenly  increased.  At  such  a  time,  the  most  rapid 
erosion  of  the  bottom  takes  place.  Conversely,  the  movement  of 
material  is  least  when  the  velocity  of  the  current  is  suddenly 
decreased,  since  the  greatest  deposit  of  sedimentary  matter  occurs 
at  that  time. 

The  sand  waves  have  an  irregular  motion  downstream,  and  the 
maximum  size  and  rate  of  progress  is  attained  when  the  stage  of 
the  river  is  at  its  highest  and  is  nearly  stationary,  their  height, 
length,  and  rate  of  motion  being  dependent  on  their  location  with 
reference  to  the  line  of  maximum  velocity.  The  waves  have  the  least 
dimensions  and  slowest  rate  of  travel  at  low  water.  They  move 
downstream  by  material  being  pushed  or  rolled  up  their  flat 
anterior  slope  and  dropped  over  their  crest,  where  it  remains 
until  the  wave  has  progressed  far  enough  downstream  to  expose 
it  again  to  the  action  of  the  current,  to  be  again  rolled  or  pushed 
forward.  The  amount  of  material  thus  moving  is  greatest  in  high 
water,  or  when  the  velocity  for  any  cause  has  been  suddenly  ac- 
celerated. Changes  in  the  form  of  waves  are  gradual,  the  waves 


28  RIVERS  AND  HARBORS 

retaining  their  form  and  individuality  as  long  as  the  velocity  of  the 
current  remains  nearly  uniform.  They  disappear  by  the  deposition 
of  sediment  carried  in  suspension,  if  the  velocity  is  suddenly 
decreased,  and  again  make  their  appearance  when  the  velocity 
approaches  uniformity  for  any  length  of  time. 

On  the  lower  Mississippi  River,  sand  waves  have  been  observed 
that  have  a  length  of  1000  feet,  a  height  of  22  feet,  and  a  maximum 
rate  of  travel  of  40  feet  per  day.  At  New  Orleans  the  amount  of 
material  transported  in  sand  waves  during  a  year  was  estimated  at 
less  than  1  per  cent  of  that  carried  in  suspension,  at  Lake  Provi- 
dence Reach  about  10  per  cent.  In  some  of  the  tributaries  of  the 
upper  Mississippi,  the  amount  rolled  along  the  river  bed  largely 
exceeds  the  amount  carried  in  suspension.  This  variation  in  the 
relation  of  material  rolled  along  the  bed  to  that  in  suspension  is 
due  to  the  relative  sizes  of  the  particles  in  different  portions  of  the 
river  bed. 

At  New  Orleans  a  large  amount  of  material  was  observed  inter- 
mittently in  suspension,  the  ratio  of  the  material  in  suspension  at 
the  water  surface  to  that  near  the  bed  being  in  some  cases  1  to 
1.83.  Furthermore,  a  difference  in  the  degree  of  saturation  of 
floods  from  the  Ohio  and  the  Missouri  rivers  respectively  could 
readily  be  observed,  notwithstanding  the  enormous  caving  of 
banks  which  occurs  during  every  flood  below  the  mouth  of  the 
Ohio  River.  At  St.  Louis  it  was  observed  that  some  of  the 
material  from  the  bed  did  not  sufficiently  mingle  with  particles 
of  water  to  remain  in  continuous  suspension  until  it  reached  the 
sea,  but  was  "  detached  for  a  time  by  some  energetic  impulse  and 
described  a  longer  or  shorter  path,  moving  in  or  out  with  the 
surrounding  water "(2). 

In  bends,  the  helicoidal  flow  impressed  upon  the  water  affects 
the  motion  of  sand  waves,  and  every  eddy  also  changes  their  form 
to  some  extent.  The  axis  of  maximum  flow  in  bends  is  diverted 
from  the  axis  of  the  channel  which  it  occupies  in  straight  reaches 
toward  the  concave  bank,  and  causes  an  erosive  action  on  that 
bank,  which  is  intensified  by  the  transverse  slope  created  in  the 
bend  combining  with  the  longitudinal  slope.  The  material 
scoured  from  the  bank,together  with  that  brought  down  the  river  in 
sand  waves,  is  diverted  from  the  direction  followed  in  straight 
reaches  to  a  diagonal  path  across  the  river.  It  forms  a  sand  bar 
extending  downstream  a  distance  from  the  origin  of  scour 


SEDIMENT  IN  NON-TIDAL  RIVERS  29 

which  is  dependent  on  the  original  slope  of  the  river  and  on  the 
radius  of  curvation  of  the  bend.  This  sand  bar  usually  forms  the 
convex  bank  throughout  the  curved  section;  but  when  the  river 
changes  its  section  to  one  that  is  straight  or  has  a  curvature  in 
the  opposite  direction,  the  intensity  of  the  helicoidal  velocity 
gradually  diminishes;  the  water  no  longer  has  sufficient  energy  to 
transport  the  material  across  the  river,  and  deposits  it  in  a  bar 
extending  across  an  unimproved  channel  in  a  diagonal  direction, 
so  that  the  length  of  its  crest  largely  exceeds  the  river's  width.  A 
dam  is  thus  soon  created  which  reacts  on  the  local  slopes  and 
velocities,  diminishing  those  above  the  bar,  and  increasing  those 
of  the  water  passing  over  it.  This  process  continues  until  the 
velocity  in  the  pool  above  the  bar  is  insufficient  to  produce  scour, 
or  until  an  equilibrium  is  established  between  the  material  brought 
to  the  bar  and  that  which  passes  over  it.  On  a  rising  river  the 
tendency  is  to  produce  the  equilibrium  by  an  elevation  of  the  crest 
of  the  bar  and  by  a  reduction  of  velocities  in  the  pool.  On  a  falling 
river  there  is  a  tendency  to  scour  a  channel  through  the  bar  and  to 
attain  the  equilibrium  by  the  resulting  increase  of  velocity  over 
it. 

While  the  movement  of  sand  waves  in  straight  reaches  is  a  slow 
process,  averaging  about  forty  feet  per  day  when  the  river  current 
has  a  mean  velocity  of  six  feet  per  second,  the  movement  of  the 
particles  of  sand  which  form  them  is  much  more  rapid,  and  the 
elevation  of  the  crest  of  the  bars  also  rapidly  increases  with  a  rise 
of  the  river  stage.  On  the  Mississippi  River  the  rise  of  some  of 
the  bars  is  at  the  rate  of  one-half  the  change  of  stage.  On  the 
Rhone  River  a  ratio  of  one  to  five  has  been  observed.  On  a  falling 
river  there  is  a  similar  scouring  of  the  bar.  The  rate  of  rise  and 
fall  of  the  crests  of  bars,  however,  varies  greatly  in  different 
reaches  of  the  same  river,  being  dependent  not  only  on  the  velocity 
of  the  water  and  the  radius  of  curvature  of  the  bend,  but  also  on 
the  eddies  formed,  on  the  character  of  the  material  moved,  and 
on  its  distribution  on  the  bar.  On  a  falling  river  the  channel  tends 
to  form  across  the  bar  along  the  line  of  the  deposited  material  which 
is  most  susceptible  to  erosion;  and  since  the  coarser  material  of 
sand  waves  is  usually  found  in  the  line  of  greatest  velocity,  the 
location  of  the  channel  across  bars  on  an  unregulated  river  usually 
differs  on  rising  stages  from  that  which  is  created  on  falling  stages. 

The  divergence  of  the  thread  of  maximum  velocity  toward  the 


30  RIVERS  AND  HARBORS 

concave  bank  of  a  bend  tends  to  produce  a  triangular  cross-section 
in  pools,  and  the  slope  of  the  concave  bank  becomes  a  function 
of  the  radius  of  curvature  and  of  the  character  of  the  soil  of  the 
bank.  If  this  soil  is  readily  eroded,  the  amount  of  material  falling 
into  the  stream  is  greater  than  the  helicoidal  flow  can  transport. 
In  that  case,  the  thalweg  depths  are  reduced,  and  gentler  slopes 
obtain,  than  those  when  material  of  greater  resistance  to  scour 
is  encountered.  In  the  fine  sediment  of  the  lower  Mississippi, 
slopes  of  one  (vertical)  to  three  (horizontal)  are  not  infrequent 
even  in  sharp  bends.  Coarse  sands  will  assume  slopes  of  one  to 
two.  If  the  concave  bank  is  composed  of  rock,  a  nearly  vertical 
slope  may  exist. 

On  a  bar  there  is  a  tendency  to  a  trapezoidal  form  until  a 
channel  is  scoured  through  the  bar  during  a  falling  river. 

The  radius  of  curvature  of  a  bend,  while  it  is  primarily  a  function 
of  the  material  that  composes  the  bank,  is  also  affected  by  the 
volume  of  the  discharge,  and  by  the  slope.  In  a  river  flowing 
through  glacial  drift,  the  variations  in  the  soil  are  the  determining 
factor  in  the  river's  course.  In  an  alluvial  valley,  however,  the 
greater  the  discharge,  and  the  gentler  the  slope,  the  longer  becomes 
the  radius  of  curvature  in  the  bends,  though  it  is  modified  by 
conditions  that  exist  in  the  bank.  Thus  in  the  Mississippi  River 
below  St.  Paul,  the  radius  of  curvature  of  the  bends  varies  from 
1500  to  4500  feet,  while  in  the  bends  above  Greenville,  Mississippi, 
it  varies  from  8000  to  15,000  feet. 

Similarly,  at  low  water  a  river  tends  to  flow  with  curves  of  less 
radius  than  in  flood  stages,  when  its  volume  has  been  very  largely 
increased.  This  tendency  to  a  change  of  curvature  at  different 
stages  has  an  injurious  effect  on  the  river's  regimen,  causing  a 
variation  in  the  location  of  the  thread  of  maximum  velocity, 
transferring  the  caving  from  one  bank  to  the  opposite  one,  and 
making  a  fill  at  high  stages  where  the  river  strives  to  create  its 
channel  during  low  water.  When  the  low-water  channel  has 
excavated  sharp  bends,  the  volume  of  water  during  floods  may  be 
too  great  to  conform  to  the  path  that  the  radius  of  curvature  strives 
to  create  and  a  large  flow  follows  a  chord  of  the  bend,  frequently  with 
sufficient  velocity  to  scour  a  secondary  channel  and  to  produce  an 
eddy  action  at  its  junction  with  the  current  along  the  bend,  which 
will  form  large  deposits  of  sediment.  A  powerful  scouring  effect 
is  also  produced  on  the  portion  of  the  bank  on  which  it  impinges. 


SEDIMENT   IN   NON-TIDAL   RIVERS  31 

An  interesting  example  of  the  influence  of  discharge  on  the 
form  of  the  river  bed  is  afforded  by  the  Atchafalaya  River  (3). 
Originally  the  Atchafalaya  was  obstructed  by  an  accumulation  of 
snags  and  drift  called  a  raft,  which  limited  the  amount  of  water 
which  could  flow  through  it  at  both  high  and  low  stages.  The 
removal  of  the  raft  and  the  construction  of  levees  along  its  banks 
has  largely  increased  its  discharge  both  at  high  and  at  low  water. 
As  a  result  the  river  has  attempted  to  enlarge  its  section,  but 
instead  of  retaining  its  old  sinuosities  and  forms  it  has  created  new 
ones,  cutting  a  channel  through  bars  on  convex  points,  and  thus 
attempting  to  adjust  its  curvature  to  its  discharge. 

In  the  Illinois  River  where  the  low-water  discharge  has  been 
increased  from  about  1000  second-feet  to  over  5000  second-feet 
by  the  flow  from  Lake  Michigan  through  the  Chicago  sanitary 
drainage  canal,  a  corresponding  change  in  the  radius  of  curvature 
of  its  bends  is  also  taking  place. 

When  a  dike  is  constructed  in  a  river,  the  resulting  disturbance 
of  the  slope  causes  the  shape  and  the  distribution  of  the  sand 
waves  to  change.  In  the  reduced  section  the  increased  slope 
scours  a  deeper  channel,  and  the  scouring  effect  is  most  intense  at 
the  end  of  the  dike.  The  eddy  below  the  dike  deposits  material 
in  its  vertex  and  has  a  gradually  reducing  scouring  effect  along  its 
outer  elements,  which  tends  to  create  a  channel  extending  from 
the  end  of  the  dike  toward  the  bank  to  which  it  is  connected.  A 
second  channel  tends  to  form  diagonally  across  the  river  toward 
the  opposite  bank,  on  account  of  the  helicoidal  motion  generated 
in  that  direction.  When  these  currents  lose  the  force  imparted 
to  them  by  the  obstruction,  a  bar  with  a  curved  crest  is  formed, 
which  incloses  both  channels.  The  navigable  channel  of  the  river 
is  determined  by  the  scour  across  this  bar,  which  occurs  on  a 
falling  river,  and  which  may  be  toward  either  bank,  dependent  on 
the  local  character  of  the  deposits  in  the  bar.  Above  the  dike,  a 
deposit  is  formed  from  sand  waves  by  eddy  action  and  from 
material  in  suspension  by  a  reduction  of  the  velocity.  Along  the 
face  of  the  dike,  there  is  a  narrow  channel  due  to  eddy  action,  if 
the  dike  makes  an  acute  angle  with  the  direction  of  flow,  and  a 
pronounced  scour  if  it  is  inclined  downstream. 

Observations  in  the  Mississippi  River  indicate  that  in  alluvial 
rivers  depths  in  pools  are  a  function  of  the  river's  discharge,  while 
depths  over  bars  vary  with  the  slope.  For  the  same  slope,  the 


32  RIVERS  AND  HARBORS 

depth  in  pools  increases  with  the  discharge.  For  the  same  dis- 
charge, the  depth  over  bars  increases  as  the  slope  diminishes. 
However,  a  slight  increase  in  the  depth  over  bars  accompanies 
an  increased  discharge. 

These  conditions  may  be  reversed,  however,  in  rivers  flowing 
through  glacial  drift,  as  is  forcibly  illustrated  by  a  comparison 
of  the  regimen  of  the  lower  Mississippi  River  with  that  of  the  St. 
Clair  River,  of  Lake  St.  Clair,  and  of  the  Detroit  River,  which 
connect  Lake  Huron  and  Lake  Erie.  In  the  lower  Mississippi 
River  wherever  steep  slopes  exist,  shoals  occur,  and  wherever  the 
slope  is  reduced  to  0.2  foot  per  mile,  a  channel  of  ample  depth 
for  navigation  exists.  In  the  connecting  waters  between  Lake 
Huron  and  Lake  Erie,  the  greatest  depths  are  formed  where 
the  slopes  are  relatively  steep,  and  when  the  slope  becomes 
less  than  0.1  foot  per  mile  the  natural  crossings  are  extremely 
shallow. 

During  storms  from  a  northerly  quadrant  a  large  amount  of 
sand,  gravel,  and  shingle  is  transported  along  the  shores  of  Lake 
Huron,  a  portion  of  which  enters  the  St.  Clair  River.  An  insignifi- 
cant amount  of  this  material  is  in  suspension.  The  sand  waves  in- 
stead of  being  propagated  along  the  river  bed  as  in  alluvial  rivers, 
enter  the  mouth  of  the  St.  Clair  River  along  its  banks,  and  contract 
the  river  instead  of  shoaling  it.  The  steep  slope  and  swift  current 
which  are  thus  created  scour  out  the  finer  material  and  pave  the 
banks  with  a  deposit  of  gravel  and  shingle  which  protects  them 
from  scour  as  efficiently  as  a  revetment.  As  the  slope  diminishes 
further  downstream,  coarse  sand  is  deposited,  in  an  enlarged 
river  section  of  less  depth.  The  finer  sands  are  carried  to  Lake 
St.  Clair,  where  the  slope  is  inappreciable,  and  a  bar  is  then 
formed  through  which  channels  originally  existed  having  depths 
varying  from  two  to  six  feet. 

At  the  foot  of  Lake  St.  Clair,  there  is  a  similar  movement  of 
sand  into  the  Detroit  River  during  northerly  storms,  which,  though 
less  in  amount  than  in  Lake  Huron,  has  been  sufficient  during 
geological  ages  to  form  a  bar  at  the  mouth  of  the  Detroit  River 
similar  to  that  at  the  mouth  of  the  St.  Clair  River. 

The  formation  of  these  lake  bars  is  similar  to  the  delta  forma- 
tions of  rivers  in  tidal  seas,  and  also  resembles  the  deposit  which 
occurs  where  an  alluvial  river  overflows  its  banks.  They  are 
highest  where  the  water  first  leaves  the  confined  bed. 


SEDIMENT   IN   NON-TIDAL   RIVERS  33 

Both  in  alluvial  rivers  and  in  those  formed  by  glacial  action, 
however,  the  pools  tend  to  form  in  the  bends  and  the  bars  in  the 
straight  reaches  between  them.  If  the  forces  acting  in  a  bend 
produce  a  diagonal  bar  in  the  reach  below  it,  and  if  the  bar  has  a 
long  crest  line  when  compared  with  the  river's  cross-section,  the 
water  flows  over  the  bar  in  a  thinner  sheet  than  it  does  when  the 
bar  is  located  more  nearly  at  right  angles  to  the  axis  of  the  channel. 
This  dispersion  of  the  water  prevents  so  great  a  scour  during  falling 
stages  as  results  from  a  more  concentrated  flow,  and  there  exists  a 
shoal  crossing  that  obstructs  low-water  navigation.  The  modern 
science  of  river  regulation  consists  in  converting  such  poor  crossings 
into  good  ones  by  so  directing  the  river  currents  as  to  cause 
the  bars  to  assume  a  position  more  nearly  at  right  angles  to  the 
axis  of  the  river  than  they  do  in  a  state  of  nature. 

From  what  precedes,  the  flow  of  a  river  may  be  summarized  as 
follows : 

In  every  river  bed  the  uplands  are  being  continuously  eroded, 
and  the  material  thus  removed  is  being  deposited  in  the  valleys 
or  transported  by  the  streams  to  a  sea  or  lake,  and  is  gradually 
being  reduced  in  size  the  further  it  is  removed  from  the  zone  of 
erosion.  In  an  alluvial  river  the  heavier  material  is  being  slowly 
and  intermittently  rolled  along  the  river  bed,  while  lighter  sands 
and  clays  are  transported  long  distances  in  suspension.  The 
water  supply  causing  these  changes  flows  over  steep  slopes  with 
great  velocity  through  the  zone  of  erosion,  but  its  slope  and  veloc- 
ity are  gradually  diminished  toward  the  river's  mouth. 

At  the  sources  of  rivers  there  are  great  variations  between  the 
high-water  and  the  low-water  discharge,  but  the  longer  the  river 
and  the  greater  the  drainage  basin,  the  smaller  the  ratio  of  the 
high-water  discharge  to  the  low-water  discharge  becomes,  although 
they  both  increase. 

Through  the  area  of  deposition,  a  river's  bed  is  gradually  rising. 
Its  channel  is  not  fixed,  but  is  liable  to  a  change  in  location  after 
every  flood.  Below  this  area,  the  river  assumes  a  sinuous  course, 
with  pools  formed  in  its  bends  and  bars  in  its  straight  reaches. 
The  crests  of  these  bars  rise  and  fall  with  the  rise  and  fall  of  the 
river.  The  slope  of  a  river  is  not  uniform,  but  is  steeper  over 
the  bars  than  in  the  pools.  The  material  carried  in  suspension 
tends  to  be  deposited  whenever  the  velocity  is  reduced.  The 
material  that  rolls  along  the  river  bed  moves  in  sand  waves,  or  is 


34  RIVERS  AND   HARBORS 

intermittently  in  suspension.  The  movement  of  the  water  is 
periodic.  The  movement  of  material  follows  the  periods  of  the 
movement  of  the  water,  but  in  place  of  being  continuous  is 
intermittent;  "its  journey  to  the  sea  is  effected  by  a  series  of 
etapes"1  (4). 

1This  expression   etapes  can  appropriately  be  translated  in  its  military  meaning:  a  day' 
march,  with  its  stoppages  for  rest  and  refreshment. 


CHAPTER  V 
A  RIVER'S   DISCHARGE  —  FLOOD   PREDICTION 

For  water  flow  in  a  conduit  a  curve  can  be  constructed  which 
gives  the  relation  between  the  height  of  the  water  surface  and  the 
discharge.  This  curve  can  be  expressed  mathematically  by  the 
equation 

2m  +1 

(A)  Q 


where  Q  is  the  discharge,  c  is  a  constant,  s  is  the  slope,  d  is  the 
greatest  depth,  and  m  is  an  exponent  varying  with  the  shape  of 
the  conduit.  The  exponent  m  is  1  when  the  sides  are  vertical, 
between  1  and  2  when  the  side-wall  is  a  curve  concave  to  the  water 
surface,  2  when  the  side-wall  is  triangular  in  shape,  and  greater 
than  2  if  the  side-wall  is  composed  of  convex  curves  that  form  a 
cusp  at  the  deepest  part  (1). 

If  the  slope  remains  uniform  at  different  stages,  the  equation 
can  be  reduced  to  the  form 

2m  +1 

(B)  Q=c'drT- 

which  represents  some  parabolic  curve.  The  exponent  of  d  varies 
with  the  shape  of  the  conduit.  Such  a  curve  is  frequently  em- 
ployed to  express  the  relation  between  the  stage  and  the  discharge 
of  a  river,  but  it  is  liable  to  give  erroneous  results  as  ordinarily  used. 
As  a  river  changes  its  stage,  its  slope  does  not  remain  constant, 
but  is  greater  on  a  rising  river  than  on  a  falling  river.  Instead 
of  having  the  parabolic  form  of  equation  (B)  shown  in  Fig.  1  by 
the  line  AB,  the  curve  assumes  the  complex  form  given  by  equation 
(A).  If  a  relation  between  height  and  slope  could  be  expressed 
mathematically,  the  relation  (A)  would  be  represented  graphically 
by  some  such  curve  as  XBY,  which  is  a  curve  of  two  branches,  one 
for  a  rising  river  and  one  for  a  falling  river.  Since  the  slope  is 
dependent,  not  on  the  actual  rise  and  fall  of  the  river,  but  on  the 
rate  of  rise  and  fall,  which  is  a  varying  quantity,  a  mathematical 
relation  between  the  stage  and  the  slope  cannot  be  obtained,  and 
the  line  XBY  merely  limits  an  area  in  which  the  discharge  for  a 

35 


36  RIVERS  AND  HARBORS 

given  height  will  be  found.  The  exact  value  of  the  discharge 
depends  on  the  rate  of  rise  or  fall. 

Not  only  is  the  slope  of  a  river  perpetually  changing,  but  also 
the  area  of  the  cross-section  of  the  river  varies  as  sand  waves  are 
propagated  downstream  in  an  unimproved  section,  or  as  the  bed 
rises  and  falls  in  a  section  that  has  been  regulated.  These  changes 
in  the  area  of  the  cross-section  occur  irregularly  and  cannot  be 
expressed  mathematically  in  terms  of  the  stage.  Hence  the  curve 
of  discharge,  instead  of  being  capable  of  representation  by  a  para- 
bolic curve,  degenerates  into  a  tangled  skein  within  the  area 
XBY,  Fig.  1.  If  numerous  discharges  are  measured  indiscrimi- 
nately on  rising  and  falling  stages,  however,  the  mean  of  the  dis- 
charge observations  will  produce  a  line  A  B,  which  should  always 
be  characterized  as  the  mean  discharge  curve. 

When  only  a  few  discharge  observations  have  been  made, 
those  at  low  stages  may  have  been  taken  on  a  rising  river  and  those 
at  high  stages  on  a  falling  river,  or  vice  versa,  producing  for  the 
mean  discharge  curve  the  line  abed,  or  the  line  a'b'c'd'.  If 
these  lines  are  extended  beyond  the  sphere  of  the  actual  observa- 
tions, as  has  often  been  done,  the  resulting  errors  are  large,  es- 
pecially if  the  curves  are  extended  as  straight  lines,  according  to 
a  practice  which  is  usual. 

A  river's  slope  may  be  affected  also  by  the  inflow  from  a  tributary 
below  the  discharge  station;  and  if  the  discharge  measurements 
are  taken  during  a  sudden  rise  or  fall  of  the  tributary,  still  greater 
perturbations  in  the  discharge  curve  occur,  as  represented  in  Fig.  1 
by  the  lines  abef  and  a'Ve'f. 

The  reader  is  cautioned  particularly  against  extending  a  mean 
discharge  curve,  no  matter  how  accurately  it  has  been  determined 
up  to  a  bank-full  stage,  to  unmeasured  discharges  at  flood  stages. 
When  a  river  overflows  its  banks,  there  is  a  violent  change  in  its 
regimen  which  will  be  reflected  in  the  mean  discharge  curve  unless 
the  river's  flow  be  restrained  by  levees. 

The  reason  that  the  slope  of  a  river  is  more  dependent  on  the 
rate  of  its  rising  and  falling  than  on  the  actual  stage  is  that  the 
river  bed  possesses  a  reservoir  capacity.  On  a  rising  river,  a 
certain  portion  of  the  flow  is  expended  in  filling  the  bed,  and  the 
maximum  discharge  at  a  lower  station  is  diminished  by  the  amount 
of  water  thus  expended.  On  a  falling  river,  the  water  thus  stored 
has  to  escape,  and  the  discharge  becomes  greater  at  the  lower 


A  RIVERAS   DISCHARGE — FLOOD   PREDICTION  37 

station  on  account  of  this  excess  flow.     Hence  the  time  required 
for  a  rise  or  a  fall  becomes  an  important  factor  in  determining 


3  £ 

Ml 

V 


v> 


li 


FIG.  1 

the  difference  in  elevation  of  the  water  surfaces  at  the  two  stations. 

If  a  river  rises  slowly,  its  slope  will  be  gentler  than  if  it  rises  rapidly. 

A  tributary  can  perform  the  work  of  filling  the  river  bed,  and 


38  RIVERS  AND  HARBORS 

thus  affect  the  slope  and  the  discharge  at  the  lower  station. 
Moreover,  if  one  rise  rapidly  follows  another  down  the  river,  the 
delay  resulting  from  the  filling  and  emptying  of  the  pools  will  cause 
the  second  rise  to  overtake  the  first  and  add  its  waters  to  it. 

On  the  long  rivers  of  the  United  States,  the  rapidity  of  the 
rise  and  fall  sometimes  has  a  large  effect  on  the  slope  and  the 
discharge.  Thus  the  crest  of  a  flood  of  fifty  feet  in  the  Ohio  River 
at  Cincinnati  may  cause  a  variation  of  from  thirty  to  fifty  feet  in 
the  heights  of  floods  at  Cairo  due  to  this  cause,  and  Humphreys  and 
Abbot  cite  a  case  where  the  maximum  discharge  of  a  flood-wave 
on  the  Mississippi  River  was  reduced  on  this  account  by  400,000 
second-feet  in  its  passage  from  Columbus,  Ky.,  to  Natchez, 
Miss.  (2). 

The  rate  of  transmission  of  a  flood  is  a  variable  quantity  that 
changes  with  the  river's  slope  and  with  the  area  of  land  subject 
to  overflow.  The  investigations  of  von  Tein  on  the  Rhine  and  its 
tributaries  would  indicate  that,  for  the  basin  of  the  Rhine,  with 
the  possible  exception  of  the  Moselle,  the  rate  of  transmission  is 
a  function  of  the  stage;  and  he  has  deduced  an  equation  for  the  time 
of  transmission  of  the  flood-wave  in  terms  of  the  stage  and  the 
distance,  which  he  applies  to  the  portion  of  the  Rhine  between 
Waldshut  and  Caub  for  stages  between  2  meters  and  5.50  meters. 
He  submits  a  table  of  the  rates  of  transmission  of  certain  flood- 
waves,  which  shows  that  the  flood  of  September,  1893,  whose 
elevation  was  220  centimeters  on  the  Waldshut  gage,  was  trans- 
mitted to  Kehl,  189  kilometers,  in  20  hours,  while  the  crest  of 
the  flood  of  June,  1876,  rising  to  667  centimeters  on  the  gage, 
required  54  hours  to  pass  over  the  same  distance  (3). 

On  the  Mississippi  River  at  midstage,  the  rate  of  propagation  of 
the  flood-wave  is  a  function  of  the  slope.  Between  Cairo  and  the 
mouth  of  White  River  (392  miles) ,  the  slope  is  about  0.4  foot  per 
mile,  and  the  time  of  transmission  of  a  flood-wave  is  about  five 
days.  From  the  mouth  of  Red  River  to  Fort  Jackson  (274  miles), 
the  slope  is  about  0.1  foot  per  mile  and  the  time  of  transmission 
is  about  one  day.  Above  a  bank-full  stage,  however,  on  account 
of  the  time  required  to  fill  and  empty  the  basins  at  the  mouths  of 
the  St.  Francis,  White,  and  Arkansas  rivers,  the  time  required 
for  the  crest  of  the  flood-wave  to  be  transmitted  from  Cairo  to  the 
mouth  of  the  White  River  varies  from  5  to  10  days,  while  it  is  not 
affected  below  the  mouth  of  Red  River,  If  a  crevasse  occurs  in 


A   RIVER'S   DISCHARGE — FLOOD   PREDICTION  39 

the  levees  of  the  St.  Francis  basin,  the  maximum  flood  at  the 
mouth  of  the  White  River  may  occur  two  weeks  after  the  flood 
passes  Cairo.  On  the  Ohio  River  the  rate  of  transmission  of  the 
flood-wave  is  about  75  miles  per  day;  on  the  Missouri  and  on  the 
Tennessee  it  is  about  100  miles  per  day. 

The  rate  does  not  exceed  4  kilometers  per  hour  on  the  Saone  or 
on  the  Seine  below  Paris,  while  it  attains  6  kilometers  per  hour 
on  the  Garonne,  and  8  to  10  kilometers  per  hour,  or  over,  on  the 
Rhone  and  on  the  Danube  (4). 

The  time  of  transmission  of  the  flood-wave  of  the  Ohio  from 
Cincinnati  to  Cairo  is  about  six  days,  but  the  floods  from  the 
Cumberland,  the  Tennessee,  and  the  upper  Mississippi  may  so 
combine  with  it  that  the  flood  attains  its  maximum  at  Cairo 
ten  or  twelve  days  after  the  crest  of  the  flood  passes  Cincinnati. 
There  can  also  be  such  a  combination  of  the  discharges  of  the 
different  rivers  that  the  maximum  flood  height  at  Cairo  will  occur 
four  days  after  that  at  Cincinnati. 

During  the  so-called  June  rise  of  the  Missouri  River,  the  dis- 
charge of  the  upper  Mississippi  frequently  determines  the  height 
of  the  flood  at  Cairo.  If  the  crest  of  a  flood-wave  from  the 
Ohio  River  arrives  at  that  locality  at  a  later  date,  it  merely  prolongs 
the  flood  stage. 

One  of  the  most  difficult  problems  the  engineer  can  be  called 
upon  to  solve  is  the  prediction  of  flood  heights.  Such  predictions 
are  required  not  only  for  the  benefit  of  navigation,  but  also  for 
agriculture.  They  are  also  a  necessity  for  the  preservation  of  the 
life  and  property  of  communities  in  valleys  subject  to  overflow. 

Many  attempts  have  been  made  to  predict  flood  heights  by 
measuring  the  rainfall  over  the  river's  basin,  and  by  computing 
therefrom  the  river's  discharge.  These  attempts  have  met  with 
little  success,  since  the  difficulties  attending  this  method  of  pre- 
diction are  practically  insurmountable.  The  extreme  variability 
of  the  rainfall  would  necessitate  the  establishment  of  an  enormous 
number  of  rain-gages  to  record  accurately  the  precipitation  over  a 
river's  basin.  The  rain  on  a  mountain  peak  differs  from  that  in 
a  valley;  that  over  forests  differs  from  that  over  cleared  land; 
that  over  a  city  differs  from  that  over  the  surrounding  country; 
and  even  the  records  of  rainfall  in  a  gage  on  the  roof  of  a  building 
may  differ  materially  from  that  of  one  established  in  a  neighbor- 
ing street-  With  measurements  of  precipitation  In  only  a  few 


40  RIVERS  AND   HARBORS 

large  cities  of  the  basin,  a  very  inadequate  conception  of  the  rain- 
fall of  the  entire  area  drained  by  the  river  is  obtained. 

The  computation  of  the  run-off  leads  to  other  difficulties.  A 
geological  survey  may  be  made  of  the  valleys  and  the  permeability 
of  the  soil  classified  under  certain  conditions.  The  conditions  are 
constantly  changing,  however.  A  rainfall  preceded  by  a  drought 
may  be  entirely  absorbed  by  the  soil,  while  if  the  ground  has  been 
saturated  by  preceding  rains,  the  run-off  may  be  a  large  percentage 
of  the  precipitation.  If  a  field  is  plowed  preparatory  to  planting 
a  crop  in  the  spring,  its  absorbing  power  largely  exceeds  that  of 
the  same  field  when  the  crop  is  being  gathered  in  the  fall.  If  a 
calm  cloudy  day  succeeds  a  rainfall,  the  amount  of  water  evapo- 
rated from  the  earth's  surface  is  much  less  than  when  the  sky  is 
clear  and  a  strong  wind  is  blowing. 

The  great  flood  of  1912  in  the  lower  Mississippi  River  was  almost 
entirely  due  to  a  moderate  rainfall  in  the  Middle  Western  States, 
which  fell  on  a  soil  which  had  become  impermeable  by  its  being 
covered  with  a  layer  of  sleet  formed  earlier  in  the  season. 

An  attempt  has  been  made  to  provide  for  these  variations  by 
means  of  a  different  coefficient  for  the  run-off  for  different  months 
of  the  year.  Thus  on  the  German  river  Main,  it  is  estimated  by 
von  Tein  that  in  January  55%  of  the  precipitation  flows  over  the 
surface,  in  February  55%,  in  March  68%,  in  April  45%,  in  May 
23%,  in  June  15%,  in  July  13%,  in  August  15%,  in  September 
17%,  in  October  20%,  in  November  30%,  and  in  December  33%. 
The  evaporation  varies  from  40%  to  55%,  the  absorption  by 
plants  from  0%  to  28%,  and  the  absorption  by  the  soil  from  0% 
to  40%.  Von  Tein  employs  these  figures  to  determine  the  amount 
of  the  rainfall  necessary  to  produce  a  flood  in  different  seasons  of 
the  year.  They  are  of  little  value  in  determining  the  height  that 
the  flood  will  attain. 

The  water  flowing  down  the  hillsides  moves  with  much  greater 
velocity  than  that  collected  in  the  swamps  and  marshes  of  the 
valley,  so  that  the  determination  of  the  percentage  of  the  precipi- 
tation that  will  enter  the  river  from  the  various  portions  of  the 
basin  at  the  same  time  becomes  a  very  intricate  problem.  The 
Burkli-Ziegler  equation  and  those  of  a  similar  nature  have  been 
deduced  from  average  conditions  in  an  area,  and  are  of  value  in 
determining  the  dimensions  of  sewers  and  drains.  But  it  cannot 
be  too  strongly  emphasized  that  floods  result  from  exceptional 


A   RIVER'S   DISCHARGE — FLOOD   PREDICTION          41 

conditions.  In  order  to  be  of  value,  a  flood  prediction  must 
differentiate  between  the  exceptional  and  the  average. 

While  it  is  impracticable  to  determine  the  absolute  height  of 
the  flood  by  the  methods  referred  to,  they  may  afford  early  in- 
formation when  a  large  flood  is  threatening,  and  for  this  purpose 
an  attempt  at  great  accuracy  is  not  desirable.  A  few  rain-gages 
in  impermeable  torrential  valleys  of  the  basin  will  give  indices 
of  the  flood  which  may  be  obscured  if  the  attempt  is  made  to 
combine  with  them  the  rain  records  of  more  permeable  portions 
with  gentler  slopes.  In  the  prediction  of  floods  in  the  Department 
of  Ardeche,  France,  where  this  method  is  employed,  a  flood  warn- 
ing is  issued  when  the  rainfall  in  48  hours  attains  over  250  milli- 
meters in  the  mountainous  valleys  (5). 

Another  method  of  flood  prediction  is  by  measuring  the  dis- 
charge at  the  origin  and  at  various  stations  established  on  the 
tributaries,  and  computing  the  discharge  at  the  lower  station  from 
these  measurements.  This  method  has  been  employed  on  the 
Elbe  (6).  It  removes  many  of  the  difficulties  resulting  from  at- 
tempting to  determine  the  discharge  from  the  precipitation. 
The  process  consists  in  determining  the  mean  discharge  curve  at 
the  different  stations  by  numerous  discharge  measurements. 
From  the  curves  are  taken  the  discharge  at  such  a  time  that  the 
sum  of  the  discharge  of  the  main  river  and  of  its  tributaries  will 
be  a  maximum  at  the  lower  stations,  and  from  its  discharge  curve 
the  height  the  flood  will  attain  is  determinined.  On  account  of  the 
reservoir  capacity  of  both  the  river  and  its  tributaries  on  the  dis- 
charge, the  proper  time  to  make  the  computation  is  difficult  to 
determine  and  it  frequently  happens  from  this  cause  that  the 
computed  discharge  at  the  lower  station  does  not  conform  to  the 
measured  discharge.  On  the  Elbe,  the  flood  is  derived  from  three 
tributaries  which  have  a  tendency  to  deliver  their  maximum  dis- 
charge to  the  main  river  simultaneously,  so  that  the  computation 
is  much  simpler  than  would  usually  obtain  in  rivers. 

A  third  method  of  flood  prediction  is  based  upon  a  study  of  the 
relations  that  exist  between  the  heights  of  the  gages  on  the  river 
and  on  its  tributaries.  This  method  seeks  to  obtain  corrections 
for  the  perturbations  caused  by  the  tributaries  and  to  add  them 
to  a  standard  flood-wave  propagated  down  the  main  stream.  On 
the  Rhine,  where  this  method  of  prediction  has  been  employed 
extensively,  there  is  first  determined  a  primary  flood-wave,  that 


42  RIVERS   AND   HARBORS 

is  to  say,  the  wave  which  would  be  produced  if  the  tributaries 
did  not  exist,  the  relations  between  the  stages  attained  by  the 
primary  flood  at  different  stations  being  shown  both  graphically 
and  by  equations  of  the  form  h2  =  ahi+b,  in  which  a  and  6  are 
constant,  while  h  is  contained  between  certain  limits. 

The  equations  employed  between  Waldshut  and  Maxau  (above 
the  Neckar  river)  are  in  meters: 

7^  =  1.01  tat  +1.28  1.72  <tat  <3.73 

tax  =0.89  tat +1-72          3.73</iwt  <4.84 
7*^  =  1.27  tat  -0.11          4.84<Awt  <6.30 

in  which  hmx  is  the  height  in  meters  at  Maxau  and  Awt  is  the  height 
in  meters  at  Waldshut. 

For  the  perturbations  caused  by  tributaries,  similar  equations 
have  been  prepared,  and  it  is  claimed  that  with  some  exceptions 
the  differences  between  the  floods  as  predicted  on  the  Rhine  and  as 
they  actually  occur  do  not  exceed  20  centimeters  (7). 

Since  the  Rhine  rises  in  the  Alps,  and  since  its  discharge  is 
regulated  by  Lake  Constance,  it  frequently  has  a  flood  when  its 
lower  tributaries  are  discharging  little  water.  Hence  the  primary 
wave  is  more  readily  determined  than  in  the  rivers  of  the  United 
States,  whose  tributaries,  flowing  at  all  stages  with  great  irregu- 
larity, interfere  with  the  primary  wave  flowing  down  the  main 
stream.  On  the  long  rivers  of  the  United  States  this  method 
would  not  give  as  satisfactory  results.  Gage  stations  on  the 
Rhine  are  less  than  20  kilometers  apart  on  the  average;  while  on 
the  Ohio  River  and  on  the  Mississippi  River  they  are  frequently 
more  than  100  miles  apart,  and  it  is  necessary  to  make  predictions 
over  river  distances  of  from  300  to  1500  miles.  An  error  of  20 
centimeters  between  stations  on  the  Rhine  may  readily  correspond 
to  an  error  of  from  10  to  20  feet  on  the  Ohio  between  Cincinnati 
and  Cairo,  since  the  principal  tributaries  of  the  Rhine  empty 
into  the  main  stream  within  a  distance  of  150  kilometers. 

On  the  Seine,  M.  Belgrand  developed  a  method  of  prediction  of 
floods  by  rises.  Reporting  on  this  method,  M.  Babinet  says, 
"It  eliminates  an  important  source  of  error  by  taking  account  of 
the  inequalities  of  the  initial  stage  on  the  gage  for  which  the  pre- 
dictions are  made.  To  this  stage  (variable  according  to  the  cir- 
cumstances which  have  preceded  the  flood  considered)  we  add  a 
probable  rise  calculated  by  the  aid  of  actual  rises  at  the  stations 
of  observation  above.  It  will  generally  be  a  function  of  the  first 


A   RIVERAS   DISCHARGE — FLOOD    PREDICTION  43 

degree  if  the  development  of  the  series  which  corresponds  to  the 
influence  of  each  gage  up  the  river  allows  of  considering  them  as 
converging  rapidly  enough"  (8). 

M.  Belgrand  selected  eight  stations  at  the  headwaters  of  the 
tributaries  of  the  Seine,  in  impermeable  valleys.  He  considered 
the  gage-readings  at  these  stations  as  indicators  of  what  was 
occurring  in  the  entire  extent  of  the  basin,  and  he  deduced  an 
empirical  law  which  gives  the  relation  of  the  rises  of  the  tributaries 
to  that  of  the  main  river.  He  found  that  the  rise  in  the  main 
river  is  twice  the  mean  of  the  total  rises  at  the  eight  stations  for 
a  normal  flood,  but  that  the  multiplier  should  be  reduced  to  1.55 
when  a  second  flood  followed  and  was  precipitated  on  one  which 
was  falling.  He  was  able  to  predict  flood  heights  at  Paris  within 
40  centimeters  three  days  in  advance  of  the  flood.  By  consider- 
ing the  inequalities  of  the  initial  stage  of  the  gage  for  which  the 
predictions  are  made,  he  recognizes  the  influence  of  the  river  slope 
on  gage-heights.  Herr  von  Tein  introduces  a  correction  for  slope 
to  a  certain  extent  by  adopting  different  equations  for  different 
stages,  but  it  is  self-evident  that  there  cannot  be  the  abrupt 
change  at  gage-readings  of  3.73  and  of  4.84  on  the  Waldshut  gage 
that  his  equations  indicate;  such  a  change  must  be  gradual. 

A  combination  of  the  methods  of  Belgrand  and  von  Tein  has 
given  good  results  at  Cairo  at  the  junction  of  the  Mississippi 
and  the  Ohio  rivers.  It  was  adopted  by  the  author  when  he  was 
assistant  to  the  Mississippi  River  Commission,  and  was  con- 
structing levees  above  Vicksburg,  Miss.  (1890-96).  By  this 
method  he  was  able  to  prepare  for  the  high-water  protection  of 
levees,  nearly  two  weeks  before  the  arrival  of  the  crest  of  the  flood, 
and  he  predicted  in  1912  that  the  flood  of  the  Mississippi  would 
exceed  in  height  all  records  at  New  Orleans,  nearly  one  month 
before  the  flood  arrived  there. 

Instead  of  attempting  to  determine  the  primary  wave  when 
there  was  no  flow  from  the  tributaries,  a  wave  representing  the 
mean  of  the  heights  at  the  different  stations  corresponding  to  a 
given  stage  at  the  origin  was  established.  The  variations  from 
this  height  due  to  changes  of  slope  and  to  perturbations  caused 
by  the  tributaries  is  computed  as  explained  in  Appendix  B.  To 
calculate  the  formulas  for  this  method  of  flood  prediction,  it  is 
necessary  to  have  gage-records  extending  over  a  period  of  at  least 
ten  years,  and  it  is  desirable  to  have  records  for  at  least  twenty  years. 


CHAPTER  VI 
RIVER  REGULATION 

The  various  methods  which  have  been  proposed  for  improving 
the  channels  of  rivers  for  navigation  may  be  classified  under  five 
heads : 

1.  By  regulation  when  the  deepening  of  the  low-water  channel 
across  the  bars  is  caused  by  works  of  contraction. 

2.  By  canalization,  which  consists  in  the  construction  of  a 
series  of  dams  across  the  river-bed,  and  overcoming  the  difference 
in  elevation  at  the  dams  by  means  of  locks. 

3.  By  dredging  and  the  removal  of  obstructions. 

4.  By  reservoirs  which  impound  the  water  during  the  high 
stages,  and  release  it  during  the  low  stages,  thus  increasing  the 
discharge  when  navigation  becomes  difficult. 

5.  By  levees  which  confine  the  flood  discharge,  and  utilize  it  to 
enlarge  the  low-water  section. 

The  great  necessity  for  obtaining  increased  depth  in  rivers  for 
navigation  has  arisen  from  the  utilization  of  the  steamboat  as  a 
propelling  power  in  place  of  animal  traction.  Conversely,  the 
substitution  of  the  steam  engine  for  the  horse  has  permitted  the 
extensive  improvement  of  our  rivers.  Prior  to  the  invention  of 
the  steamboat,  the  flat-boat  and  the  raft  which  were  employed 
for  transporting  merchandise  usually  floated  downstream  with 
the  current;  but  in  ascending  a  river,  animal  traction  was  required, 
for  which  purpose  a  tow-path  was  built  close  to  the  bank  and  the 
boats  were  towed  by  horses,  since  the  horse  is  a  more  economical 
source  of  power  than  a  man  handling  an  oar  or  a  pole. 

The  first  efforts  to  improve  navigation  consisted  in  bank 
revetment.  This  was  done  not  only  to  prevent  the  land  from 
caving  into  the  river,  but  also  to  preserve  the  tow-path.  When 
bank  protection  was  afforded  by  projecting  dikes,  a  longitudinal 
dike  was  frequently  constructed  close  to  their  channel-ends  as  a 
tow-path.  Many  of  the  longitudinal  dikes  of  the  Rhine  river 
were  built  originally  for  this  reason. 

When  a  sand  bar  obstructed  navigation  a  longitudinal  dike  was 
constructed  on  one  side  of  the  channel  to  serve  as  a  tow-path. 

44 


RIVER   REGULATION  45 

A  second  longitudinal  dike  was  built  near  the  opposite  bank  and 
connected  to  it,  sometimes  by  a  perpendicular  spur-dike,  but  fre- 
quently by  a  dike  which  gave  a  funnel  shape  to  the  river  channel. 
The  distance  between  the  dikes  was  computed  by  the  ordinary 
hydraulic  formulas,  so  as  to  give  the  desired  depth  for  the  low-water 
discharge  with  the  local  slope  existing  over  the  bar  before  the  im- 
provement was  inaugurated. 

When  low-water  depths  not  exceeding  three  feet  were  desired 
over  gravel  bars,  this  method  of  improvement  by  parallel  dikes 
gave  fairly  satisfactory  results.  Frequently,  however,  greater 
scour  would  occur  than  was  anticipated.  The  upper  pool  would 
be  lowered,  and  a  sand  bar  would  be  formed  by  the  eroded  material 
below  the  improvement.  By  the  extension  of  the  dikes  so  as  to 
include  in  the  contracted  channel  the  new  bar  which  had  formed, 
a  gentler  slope  would  be  created  over  the  crossing,  and  the 
tendency  to  excessive  scour  on  the  bar  would  be  checked.  The 
resulting  slight  lowering  of  the  upper  pool  did  not  seriously 
affect  the  slopes  on  the  bar  above.  The  Meuse  River  (1)  in  France, 
the  upper  Tennessee  (2),  and  the  Gasconade  River  (3)  afford 
examples  of  this  method  of  improvement. 

With  the  introduction  of  steam  power,  greater  depths  were 
required  in  river  channels  to  meet  the  competition  resulting  from 
the  reduced  cost  of  transportation  on  land.  When  it  was  at- 
tempted to  increase  the  channel  depths  in  a  river  whose  bars  had 
been  deepened  by  parallel  dikes,  serious  difficulties  were  en- 
countered. A  further  contraction  across  the  bar  became  neces- 
sary, and  this  involved  the  construction  either  of  a  new  longi- 
tudinal dike  or  of  a  system  of  spur-dikes  extending  into  the  channel 
from  the  dike  opposite  the  tow-path.  The  increased  velocity 
generated  caused  the  erosion  even  of  gravel.  An  abnormal  scour 
occurred  across  the  bar,  large  deposits  were  formed  in  the  pool 
below,  and  an  excessive  lowering  of  the  pool  above  diminished  the 
depths  across  its  upper  bar. 

It  was  then  attempted  to  modify  the  slopes  in  the  pools,  re- 
ducing their  curvature  as  much  as  possible  by  the  construction 
of  longitudinal  dikes  of  gentle  curvature  along  their  concave  banks, 
and  building  submerged  sills  across  them.  The  purpose  was  to 
give  the  river  as  straight  a  course  as  practicable  with  a  uniform 
slope,  in  place  of  the  natural  sinuous  course  with  gentle  slopes 
through  pools  and  steep  slopes  on  bars. 


46  RIVERS  AND   HARBORS 

The  tow-path  formerly  had  an  importance  on  European  rivers 
that  it  never  attained  in  the  United  States.  It  could  readily  be 
maintained  along  a  river  thus  straightened,  but  the  flowing  water 
was  not  as  amenable  to  the  change.  The  river  through  geological 
eons  had  been  adjusting  its  length,  its  curves,  and  its  slopes 
through  pools  and  over  bars  to  the  character  of  the  soil  through 
which  it  flowed  and  of  the  material  which  it  was  transporting, 
and  it  had  to  adapt  itself  to  the  new  conditions.  The  straighten- 
ing of  the  river  would  reduce  its  length  and  therefore  increase  its 
slope.  The  slope  through  the  pools  would  be  further  augmented 
by  the  change  in  the  relation  of  the  slope  on  the  bars  to  the  slope 
through  the  pools,  and  by  the  contraction  of  the  area  of  the 
waterway.  The  transverse  slope  of  the  pool  would  become  re- 
duced by  the  straightening,  the  longitudinal  velocity  become 
largely  increased,  and  the  bed  rapidly  deepened  even  when  the 
revetment  of  the  bend  preserved  its  bank  from  erosion.  Unless 
the  submerged  sills  were  so  numerous  as  practically  to  convert 
the  river  bed  into  a  paved  conduit,  this  deepening  frequently 
became  so  great  as  to  undermine  the  revetment  and  to  destroy  it, 
and  if  the  revetment  was  of  the  mattress  type  extensively  em- 
ployed in  the  United  States,  the  material  on  which  it  rested  tended 
to  slide  into  the  river.  If  there  was  a  longitudinal  dike  of  rip-rap, 
so  frequently  found  as  a  bank  protection  on  European  rivers,  a 
serious  settling  occurred. 

The  material  thus  eroded  would  be  added  to  the  sand  waves 
that  were  being  transported  down  the  river,  and  be  slowly  moved 
through  the  contracted  section,  increasing  the  height  of  the  sand 
waves  during  rising  stages  of  the  river;  and,  during  falling  stages, 
channels  usually  of  insufficient  depth  for  navigation  became 
scoured  through  them. 

These  sand  waves  following  one  another  through  the  con- 
tracted section  produced  below  it  a  bar  which  raised  the  water 
surface,  and  combining  with  the  lowering  of  the  water  surface  in 
the  pool  above,  created  a  slope  sufficiently  gentle  to  reduce  the 
scour  to  normal  proportions.  The  mean  slope  through  the  improved 
section  finally  became  gentler  than  that  of  other  portions  of  the 
river,  and  the  channel  was  locally  improved,  but  at  the  expense  of 
the  slopes  and  depths  at  other  localities.  Since  the  total  fall  in  a 
river  from  its  source  to  the  sea  is  a  fixed  quantity,  a  diminution  of 
slopes  in  one  section  must  be  accompanied  by  an  increase  at  others. 


RIVER   REGULATION  47 

M.  Girardon  cites  as  an  example  of  the  injurious  effects  of 
straightening  a  river,  the  Canal  de  Miribel  on  the  Rhone.  An 
excellent  example  is  also  afforded  by  the  improvement  of  the 
Mississippi  River  in  front  of  the  city  of  St.  Louis.  On  account 
of  the  commercial  importance  of  that  city,  the  river  channel  was 
so  constructed  as  to  follow  the  Missouri  bank,  within  the  city 
limits.  This  contraction  and  straightening  has  reduced  the  slope 
to  0.2  foot  per  mile  on  a  river  which  normally  has  an  average  slope 
of  0.6  foot  per  mile,  and  an  excellent  channel  exists  in  front  of  the 
city,  but  the  slopes  on  the  Chain  of  Rocks  immediately  above 
have  been  injuriously  affected  and  annual  dredging  is  required 
on  the  crossings  below  the  city. 

At  the  Sixth  International  Navigation  Congress  in  1894,  M. 
Girardon  presented  a  project  for  improving  the  river  Rhone,  in 
which  (instead  of  attempting  to  straighten  the  river  and  equalize  its 
slopes) ,  he  proposed  to  allow  it  to  pursue  its  natural  sinuous  course, 
with  gentle  slopes  in  the  pools  and  steeper  ones  on  the  bars,  and  to 
create  the  improvement  by  converting  poor  crossings  wherever  they 
occurred  into  good  ones.  The  result  was  to  be  brought  about  by 
giving  a  suitable  direction  to  the  river  currents  across  the  bars. 

According  to  M.  De  Mas,  this  suggestion  of  M.  Girardon  caused 
a  revolution  in  river  regulation.  When  combined  with  the  in- 
vestigations of  M.  Fargue  on  The  Influence  of  Bends  on  Channel 
Depths,  it  led  to  an  entirely  new  conception  of  the  proper  functions 
of  the  works  constructed  to  improve  channels.  The  direction 
which  the  works  of  contraction  gave  to  the  confined  waters  be- 
came of  more  importance  than  the  relative  amount  of  contrac- 
tion; and  since  the  steamboat  had  finally  displaced  animal  trac- 
tion, the  river  channel  became  free  from  bank  control,  and  could  be 
given  such  a  direction  as  to  produce  the  proper  effect  on  the  bars. 
This  method  of  river  regulation  may  be  called  the  French  method, 
as  distinguished  from  river  straightening,  of  which  the  ablest  ex- 
ponents were  German  authors. 

When  this  method  is  followed,  the  sinuous  course  of  the  river 
is  preserved,  or  even  intensified,  so  as  to  render  more  stable  the 
location  of  the  bars.  No  attempt  is  made  to  obtain  uniform 
depths,  nor  uniform  slopes.  The  bars  are  permitted  to  form  in 
the  crossings  and  their  crests  to  rise  with  a  rise  in  the  river  stage, 
but  during  a  falling  stage  the  currents  are  given  such  a  direction 
as  to  scour  a  channel  of  the  desired  depth  across  them. 


48  RIVERS  AND   HARBORS 

The  curved  trace,  instead  of  permitting  sand  waves  to  move 
unrestrained  along  the  river  bed,  removes  them  from  the  pools,  and 
permanently  locates  them  in  the  crossings.  By  giving  a  proper 
curvature  to  the  bends,  the  obliquity  of  the  bars  to  the  axis  of 
the  channel  is  diminished,  and  the  volume  of  the  flow  across 
them  is  concentrated  to  the  extent  necessary  to  produce  suitable 
channel  depths.  The  most  characteristic  difference  in  the  two 
systems  of  river  improvement  is  shown  in  the  use  of  ground  sills. 
In  the  German  system,  the  ground  sill  was  used  generally  to 
modify  the  slopes  in  the  pools.  M.  Girardon  employs  it  to  restore 
the  steeper  slope  of  the  crossing,  and  to  preserve  the  pool  level, 
if  by  any  means  the  scour  has  become  too  great  for  the  natural 
formation  of  a  bar  at  that  locality. 

The  investigations  of  M.  Fargue l  have  been  presented  in  the 
form  of  the  following  six  laws: 

"I.  THE  LAW  OF  DEVIATION.  The  deepest  and  the  shallowest 
points  in  the  channel  are  below  the  vertex  and  the  ends  of 
the  curve,  respectively.  On  the  Garonne,  the  displacement  is 
about  one-fifth  the  length  of  the  curve,  but  is  less  than  that 
on  the  shallower  crossings. 

"II.  THE  LAW  OF  GREATEST  DEPTH.  The  point  of  maximum 
depth  is  the  deeper  as  the  curvature  of  the  vertex  of  the  curve  is 
sharper.  The  relation  between  depth  and  curvature  on  the 
Garonne  is  given  approximately  by  the  formula 

C  =  0.03  H*  -  0.23  H2 + 0.78  H-  0.76, 

where  C  represents  the  reciprocal  of  the  radius  of  curvature  at 
the  vertex  of  the  curve  expressed  in  kilometers  and  H  is  the 
low-water  depth  at  the  deepest  point  of  the  channel  expressed 
in  meters. 

"III.  THE  LAW  OF  THE  TRACE.  In  the  interest  of  both  the 
average  and  maximum  depths,  the  curves  should  neither  be  too 
short  nor  too  long.  On  the  Garonne,  this  length  is  preferably 
about  one  and  one-third  kilometers. 

"IV.  THE  LAW  OF  THE  ANGLE.  For  equal  lengths  of  curve, 
the  average  depth  of  the  pool  is  the  greater  as  the  central  angle 

Etude  sur  la  Correlation  entre  la  Configuration  du  Lit  et  la  Profondeur  d'Eau  dans  les 
RiviSres  &  Fond  Mobile,  par  M.  Fargue,  Ingenieur  des  Fonts  et  ChaussSes,  Annales  des  Fonts 
et  Chaussees,  Memoires  et  Documents,  1868,  p.  34. 


RIVER  REGULATION  49 

subtended  by  the  curve  is  the  smaller.  The  equation  derived 
from  this  analysis  on  the  Garonne  is 

#  =  1.50+\/<72+1.71C 

in  which  H  represents  the  average  depth  of  pool  at  low  water 
in  meters,  and  C  the  reciprocal  of  the  average  radius  of  curva- 
ture in  kilometers. 

"V.  THE  LAW  OF  CONTINUITY.  The  longitudinal  channel 
profile  shows  gradual  variations  only  when  the  curvature 
changes  gradually.  Abrupt  modifications  of  depth  accompany 
rapid  variations  in  curvature. 

"VI.  LAW  OF  THE  SLOPE  OF  THE  BED.  If  the  curve  varies 
continuously,  an  increasing  radius  of  curvature  marks  a  reducing 
depth  and  a  decreasing  radius  of  curvature  is  accompanied  by 
a  deepening.  The  rate  of  change  in  depth  which  is  the  slope  of 
the  bed  is  approximately  represented  on  the  Garonne  by  the 
equation: 

Q=0.1553p+0.0114p3, 

when  Q  is  the  variation  per  kilometer  of  the  reciprocal  of  the  grad- 
ually changing  radius  of  curvature  expressed  in  kilometers,  and 
p  is  the  variation  per  kilometer  of  the  depth  given  in  meters  (4)." 
In  the  improvement  of  European  rivers,  a  longitudinal  dike 
frequently  is  constructed  in  bends.  As  such  a  dike  can  be  given 
any  desired  form,  there  has  been  considerable  discussion  as  to 
whether  it  should  be  constructed  in  the  curve  of  a  parabola,  a 
lemniscate,  or  a  sinusoid.  The  arc  of  a  circle  extended  by  tangents 
across  the  bars  conflicts  with  Fargue's  laws.  In  the  United  States, 
there  is  not  the  same  latitude  in  selecting  the  proper  curve,  since 
bank  revetment  by  mattress  construction  has  been  substituted 
almost  universally  for  the  longitudinal  dike  in  the  protection  of 
bends.  This  construction  must  follow  the  sinuosity  caused  by 
the  natural  caving  of  the  bend,  but  even  with  this  method  of 
construction  there  is  considerable  latitude  in  the  direction  to  be 
given  to  the  currents,  since  fortunately  the  caving  of  a  bend  only 
exceptionally  conforms  to  the  arc  of  a  circle,  and  the  point  of 
divergence  of  the  current  from  the  bend  is  optional,  to  a  certain 
extent,  with  the  engineer. 

To  comprehend  the  importance  of  Fargue's  laws,  let  us  as- 
sume that  a  river  flowing  along  the  bluff  which  limits  one  side 
of  a  valley  is  diverted  by  some  cause  to  the  opposite  bluff,  that  it 


50 


RIVERS  AND   HARBORS 


leaves  one  bluff  and  approaches  the  other  in  circular  curves,  and 
that  it  is  required  to  improve  the  crossing  between  them.  The 
extension  of  the  bank  by  dikes  on  lines  tangent  to  the  curves,  as 
shown  by  broken  lines  in  Fig.  2,  is  forbidden  by  Fargue's  laws,  and 
the  reason  is  obvious  from  a  little  investigation. 

In  the  curve  created  as  the  river  leaves  the  bluff,  a  steep  trans- 
verse slope  is  produced  with  a  resulting  helicoidal  movement  of 
the  water,  which  causes  the  channel  to  "hug  the  concave  bank," 


B    A 


•B 


FIG.  2 

as  river  pilots  say.  The  sand  scoured  from  the  channel  is  de- 
posited on  the  convex  side,  at  a  distance  from  its  point  of  departure 
varying  with  the  longitudinal  slope  of  the  river,  and  the  sharpness 
of  curvature  of  the  bend. 

While  the  tangent  to  the  curve  has  no  power  to  produce  a 
transverse  slope,  it  will  conserve  for  long  distances  one  that  is 
already  created,  and  the  thalweg  will  continue  to  hug  the  dike 
tangent  to  the  concave  bank  until  the  helicoidal  force  created  by 


RIVER  REGULATION  51 

the  transverse  slope  has  exhausted  its  energy.  There  will  also  be 
a  deposit  of  sediment  on  the  opposite  side  of  the  river,  which 
will  form  closer  to  the  channel  as  the  energy  is  diminished  and 
will  finally  attach  itself  to  the  dike  near  the  point  where  the 
transverse  slope  becomes  again  horizontal.  This  will  result  in 
the  formation  of  a  long  diagonal  bar  DE  across  the  river  during 
rising  stages  (Fig.  2). 

As  the  river  falls  the  water  flows  over  this  bar  in  a  thin  sheet, 
with  no  concentration  of  force  at  any  particular  point,  and  it 
finally  scours  through  it  at  some  point  as  C  or  C",  where  finer 
material  was  deposited  accidentally  during  the  rising  river.  Such 
conditions  usually  produce  a  shoal  and  crooked  channel  difficult 
to  navigate  at  low  water.  If,  however,  the  bank  curves  so  as  to 
produce  a  reduced  degree  of  curvature  below  the  vertex  of  the 
curve,  instead  of  the  circular  form  first  assumed,  or  if  a  dike  is 
constructed  as  shown  by  the  full  lines  of  Fig.  2,  the  transverse 
slope  created  at  the  vertex  of  the  bend  has  an  opportunity  to 
flatten  out,  and  the  line  of  deepest  water,  instead  of  remaining 
close  to  the  dike,  tends  to  return  to  the  middle  line  of  the  channel. 
The  helicoidal  flow  diminishes  and  the  longitudinal  flow  along  the 
convex  bank  increases. 

The  bar  across  the  channel  leaves  the  dike  connected  to  the 
convex  bank  further  downstream,  and  attaches  itself  to  the 
opposite  dike  further  upstream,  creating  a  shorter  crest  over  which 
the  currents  have  a  greater  erosive  effect  while  the  river  is  falling. 
The  thalweg  follows  a  straighter  line  and  is  deeper  and  much 
easier  to  navigate  at  low  stages. 

As  the  river  approaches  the  curve  at  the  lower  bluff,  there  is 
a  reaction  which  also  merits  consideration.  On  account  of  the 
inertia  of  the  water,  the  transverse  slope  which  is  created  there 
cannot  be  formed  suddenly.  If  a  dike  tangent  to  a  circular  arc 
extends  upstream,  a  gradually  diminishing  transverse  slope  is 
propagated  along  it  by  the  reaction  similar  to  the  one  trans- 
mitted downstream  along  the  tangent  to  the  curve  at  the  upper 
bluff. 

This  creates  a  gradually  increasing  helicoidal  motion  as  one  ap- 
proaches the  lower  bend,  with  the  result  that  the  thalweg  also 
hugs  the  tangent  to  the  lower  curve  and  will  not  conform  in  direction 
with  the  one  above  the  bar.  The  scour  across  the  bar  will  tend  to 
produce  a  river  cross-section  shown  by  the  broken  line  AB  in 


52  RIVERS  AND   HARBORS 

Fig.  2,  with  a  mobile  channel  across  the  bar  approximately  at 
right  angles  to  those  in  the  upper  and  lower  pools.  It  is  therefore 
necessary  to  modify  the  curve  of  approach  to  the  lower  vertex 
in  a  manner  similar  to  that  ngflessary  after  leaving  the  vertex 
of  the  upper  bend,  thus  forming  the  sinusoidal  curves  advocated 
by  M.  Fargue. 

Not  only  are  proper  curves  necessary  in  the  upper  and  lower  pools, 
but  also  their  lengths  must  be  such  that  both  thalwegs  will  pass 
through  what  M.  Fargue  calls  the  point  of  inflection  for  curves 
turning  in  opposite  directions,  or  the  point  of  surflection  when 
the  curves  continue  in  the  same  direction.  This  leads  to  the  law 
of  the  trace  that,  in  the  interest  of  both  the  average  depth  and 
the  maximum  depth,  the  curves  should  neither  be  too  short  nor 
too  long.  Their  proper  length  is  a  direct  function  of  the  diff- 
erence in  level  on  the  opposite  banks  of  the  bends,  and  an  inverse 
function  of  the  river's  longitudinal  slope.1 

In  the  case  assumed,  the  location  of  the  curves  was  fixed  by 
the  configuration  of  the  ground.  When  the  natural  curves  are 
too  short  or  too  long,  the  problem  of  overcoming  this  condition 
presents  itself.  If  the  curves  are  too  long,  the  difficulty  fre- 
quently can  be  obviated  by  introducing  additional  bends.  If  the 
curves  are  too  short,  the  slope  over  the  crossing  will  be  destroyed, 
the  low-water  surface  will  be  permanently  lowered,  and  it  can  be 
restored  only  by  the  construction  of  submerged  sills,  which  will 
limit  the  depth  during  low  stages. 

By  creating  proper  curves  in  bends  and  by  extending  them  by 
dikes,  the  French  method  of  river  regulation  seeks  to  give  a  proper 
direction  to  the  river  currents  so  as  to  obtain  suitable  depths  on 
the  crossings;  the  material  scoured  from  the  channel  is  deposited 
on  the  banks  opposite  the  dikes  in  such  a  manner  as  to  create 
proper  widths  both  across  the  bars  and  in  the  bends ;  and  according 
to  the  investigations  of  M.  Fargue,  the  river  assumes  a  greater 
width  in  the  bends  than  on  the  crossings.  This  assumes  that  the 
filaments  of  flowing  water  have  the  same  radius  of  curvature  as  the 
dikes  they  follow.  Such  an  assumption,  however,  can  be  true  only 
for  certain  stages.  During  a  flood,  a  river's  discharge  is  ordinarily 

1  Figure  2  is  inserted  to  illustrate  a  general  principle,  and  is  not  drawn  to  scale,  since  this 
would  necessitate  further  assumptions  in  reference  to  the  distances  and  the  slopes.  Under 
certain  conditions,  the  contraction  by  the  tangents,  instead  of  permanently  locating  the  bar 
along  the  line  DE,  would  induce  a  motion  of  sand  waves  through  the  reach,  and  the  cross- 
section  AB  of  the  bed  could  be  maintained  only  by  submerged  dikes. 


RIVER   REGULATION  53 

from  20  to  50  times  that  of  low  water,  and  the  water  tends  to  flow 
with  a  much  larger  radius  of  curvature.  The  thread  of  maximum 
velocity  may  then  have  a  gentler  curvature  than  the  bend,  and 
may  impinge  upon  the  dike  at  some  point  lower  in  the  bend  than 
the  vertex.  The  filaments  of  water  are  then  forced  by  the  dike 
into  a  change  of  direction  with  a  sharp  radius  of  curvature. 

Above  this  point  there  is  a  tendency  to  eddy  action,  which  may 
fill  the  channel  scoured  at  lower  stages,  and  a  channel  may  scour 
across  a  chord  of  the  bar  on  the  convex  bank. 

Below  the  new  vertex  thus  formed,  the  dike  ceases  to  conform  to 
M.  Fargue's  law  of  continuity,  and  the  obliquity  of  the  bar 
below  to  the  center  line  of  the  channel  is  affected.  To  reduce 
these  disadvantageous  conditions  as  much  as  possible,  the  works 
of  regulation  are  given  a  low  elevation,  so  that  the  great  mass  of 
the  flood-waters  will  pass  over  them. 

On  the  Rhone  (5),  the  elevation  of  the  curved  dike  is  one 
meter  above  low  water  at  its  vertex,  gradually  diminishing  to  the 
elevation  of  low  water  at  its  extremities.  In  the  latest  project 
for  the  improvement  of  the  Po  (6),  it  is  proposed  to  give  longitu- 
dinal dikes  an  elevation  of  one  meter  above  low  water. 

To  protect  the  longitudinal  dikes  from  scour  behind  them  during 
floods,  it  is  necessary  to  join  them  to  the  bank  by  means  of  spur- 
dikes  which  have  a  slope  from  the  bank  to  the  dike  with  the  same 
elevation  as  the  dike  at  the  point  of  junction.  To  prevent  the 
current  during  floods  from  scouring  across  the  points  of  bars,  a 
series  of  spur-dikes  on  the  convex  banks  of  bends  is  necessary. 
These  are  given  as  low  an  elevation  as  practicable  at  their  channel 
ends  so  as  to  interfere  with  the  flood  discharge  as  little  as  possible, 
and  are  made  to  slope  gradually  upwards  towards  the  bank,  so  that, 
as  the  stage  falls  after  a  flood,  the  river  will  gradually  revert  to  the 
regulated  form  that  has  been  given  it  at  low  water. 

While  the  dimensions  of  the  channels  across  the  bars  are  care- 
fully computed,  reliance  is  placed  on  ground  sills  and  spur-dikes 
to  maintain  the  desired  channel,  and  submerged  dikes  are  fre- 
quently employed  to  assist  the  longitudinal  dikes  in  giving  a 
proper  direction  to  the  river  currents.  In  fact,  reliance  is  placed 
on  dike  construction  to  correct  any  errors  that  may  have  been 
committed  in  the  mathematical  computations.  By  these  means, 
the  crossings  are  given  a  fixed  position  with  a  scour  over  them  in 
a  proper  location  during  falling  stages. 


54  RIVERS  AND   HARBORS 

While  the  French  method  of  river  regulation  is  considered  the 
proper  one,  the  student  is  cautioned  that  river  improvement  is 
not  susceptible  of  the  rigid  mathematical  analysis  which  obtains 
in  certain  other  engineering  fields,  such  as  bridge  construction  for 
example.  The  principles  are  immutable,  but  the  conditions  that 
exist  in  nature  are  not  susceptible  of  mathematical  expression 
with  our  present  knowledge.  Each  river  has  its  own  equation. 

Even  the  process  of  river  straightening  which  Fargue's  laws 
condemn  has  been  applied  successfully  to  the  lower  Rhine  River, 
where  low-water  depths  of  three  meters  are  maintained  with  a 
reasonable  amount  of  dredging.  Herr  Jasmund  (7)  calls  atten- 
tion to  the  fact  that  in  former  days  the  lower  Rhine  was  given  an 
abnormal  degree  of  curvature  by  the  enlargement  of  the  islands, 
which,  were  crown  lands.  The  increase  in  size  of  the  islands  was 
accompanied  by  a  caving  of  the  adjacent  bends,  and  natural 
conditions  have  now  been  restored  by  the  works  of  river  regula- 
tion. On  the  upper  Rhine,  river  straightening  has  produced  a 
lowering  of  the  river,  but  M.  Coblenz  states,  in  a  description  of 
the  Rhine,  that  notwithstanding  the  straightening  of  numerous 
bends  between  Mannheim  and  Mayence,  the  slope  and  the  velocity 
are  still  extremely  feeble.  He  adds  that,  in  the  project  for  the 
improvement  of  the  river  above  Strasburg,  a  sinuous  course  for 
the  low-water  channel  has  been  adopted  (8). 


CHAPTER  VII 
DIKE  CONSTRUCTION  AND  BANK  PROTECTION 

A  rigid  adherence  to  the  French  system  of  river  regulation 
would  require  a  longitudinal  dike  along  the  concave  bank  of  a 
sinusoidal  form  and  of  low  elevation  to  give  proper  direction  to 
the  low-water  flow.  This  dike  would  have  to  be  connected  to 
the  bank  by  a  system  of  spur-dikes  to  prevent  scour  behind  it, 
and  a  system  of  similar  dikes  on  the  convex  bank  would  have  to 
be  built,  to  prevent  scour  of  that  bank  during  high  stages.  These 
dikes  would  be  given  as  low  an  elevation  as  practicable  at  the 
limiting  line  of  the  low-water  channel,  and  would  be  made  to 
slope  gradually  upwards  as  they  recede  from  it,  so  as  to  bring  the 
river  back  to  the  low-water  channel  on  declining  stages.  The 
crossings  also  must  be  protected  by  spur-dikes  to  prevent  the  high 
stages  from  producing  an  injurious  scour. 

When  the  low-water  discharge  of  a  river  is  small  and  the  maxi- 
mum depth  on  bars  must  be  attained  to  provide  the  depths  re- 
quired by  navigation,  a  rigid  adherence  to  these  requirements 
may  be  necessary.  In  the  United  States,  the  low-water  discharge 
of  most  rivers  that  are  being  regulated  is  fortunately  large,  and 
the  projects  for  improvement  of  these  rivers  do  not  require  the 
maximum  practicable  development  of  the  depth  on  bars.  This 
fact  permits  a  considerable  latitude  in  the  curvature  which  can 
be  given  to  the  bends,  and  economical  considerations  have  there- 
fore led  to  a  marked  difference  between  the  methods  of  construc- 
tion employed  in  river  regulation  in  the  United  States  and  those  at 
present  prevailing  on  most  European  rivers. 

The  excessive  cost  of  a  longitudinal  dike  of  rip-rap,  in  the  depths 
found  in  the  bends  of  the  large  rivers  of  the  United  States,  almost 
precludes  its  employment  for  bank  protection,  not  only  on  account 
of  the  expenditure  in  its  original  construction,  but  also  on  account 
of  the  large  amount  of  money  required  for  maintenance  due  to 
the  great  scour  that  the  dike  creates  in  the  river  bed.  Conse- 
quently, there  has  been  substituted  very  generally  a  bank  revet- 
ment consisting  of  a  subaqueous  brush  mat  covered  with  a  sufficient 
amount  of  stone  to  hold  it  in  place,  and  an  upper  bank  pavement 

55 


56  RIVERS  AND   HARBORS 

consisting  either  of  rip-rap,  of  stone  more  carefully  laid,  or  of 
concrete. 

The  extensive  growth  of  willows  on  the  bars  of  our  western 
rivers  furnishes  a  cheap  material  for  the  construction  of  the  mats, 
which  also  can  be  employed  economically  in  many  localities  to 
form  the  body  of  the  spur-dikes  on  the  opposite  bank.  The 
substitution  of  the  revetted  bank  for  the  longitudinal  dike  neces- 
sitates the  utilization  of  the  natural  curves  created  by  caving, 
instead  of  the  theoretically  perfect  form  which  can  be  given  to 
the  dike.  There  is  a  resultant  loss  in  the  scouring  power  over 
the  crossing  below,  but  this  loss  is  justified  by  reasons  of  economy 
if  the  depth  obtained  across  the  bar  exceeds  the  limits  prescribed 
by  the  project.  There  still  remains,  however,  considerable  latitude 
in  determining  the  direction  to  be  given  to  the  river  in  its  passage 
from  one  bank  to  the  other. 

The  large  amount  of  sediment  carried  in  suspension  in  many 
American  rivers  has  also  led  to  the  substitution  of  permeable 
dikes  instead  of  spur-dikes  of  gravel  or  stone.  These  dikes  con- 
sist of  one  or  more  rows  of  piles,  strongly  braced,  on  which  was 
originally  suspended  a  wattled  screen  of  willow  brush,  to  check 
the  current  and  cause  a  deposit  of  the  sediment.  In  the  turbid 
waters  of  the  Missouri  and  the  middle  Mississippi,  the  screen  is 
now  omitted  as  the  piles  themselves  have  been  found  to  check 
the  flow  sufficiently  to  cause  a  large  deposit.  The  piles  are  spaced 
about  two  feet  apart  in  two  or  more  rows  in  quincunx  order  and 
are  bound  together  in  groups  of  three.  This  forms  a  structure 
of  considerable  resisting  power  to  floating  ice  or  debris. 

By  means  of  such  spur-dikes  the  banks  are  raised  by  river  de- 
posits to  any  elevation  below  a  bank-full  stage  that  may  be  de- 
sired, and  there  is  no  reason  why  the  same  method  could  not  be 
employed  to  induce  deposits  during  flood  stages  if  it  were  con- 
sidered necessary.  There  is,  however,  a  practical  limitation  to 
the  height  of  such  dikes  in  rivers  that  carry  a  large  amount  of 
drift  during  floods  or  of  ice  during  freezing  weather.  If  there  is 
not  a  sufficient  deposit  of  sediment  before  the  drift  or  ice  can 
accumulate  above  the  structure  in  quantity,  the  dike  may  be 
destroyed  by  lateral  pressure.  Thus,  in  a  river  in  which  the  de- 
posits between  permeable  dikes  can  readily  attain  the  mid-stage 
height  during  a  single  rise,  the  attempt  sometimes  is  made  to  raise 
them  to  such  a  height  that  the  made  ground  can  be  used  for  agri- 


DIKE    CONSTRUCTION  AND    BANK   PROTECTION          57 

cultural  purposes.  This  will  require  the  action  of  several  floods. 
Before  this  can  be  accomplished,  the  tops  of  the  dikes  will  retain 
a  mass  of  floating  debris  which  may  exert  enough  lateral  pressure 
on  the  piles  composing  them  to  cause  their  destruction. 

The  French  method  of  regulation,  which  requires  low  dikes  at 
the  limiting  line  of  the  low-water  channel,  gradually  increasing 
in  height  as  the  dike  recedes  from  this  line,  tends  to  minimize 
this  danger,  since  the  greatest  accumulation  of  drift  usually  takes 
place  at  the  outer  end  of  the  dikes,  and  will  pass  over  low  dikes 
as  the  river  rises  to  high  stages. 

When  stone  is  abundant,  as  on  the  middle  Mississippi  River, 
the  drift  can  be  sunk  as  it  forms  against  the  dike,  and  instead  of 
destroying  it,  increase  its  stability. 

On  the  upper  Missouri  River,  there  has  recently  been  substituted 
for  the  permeable  dike  the  retard,  which  consists  of  a  mass  of 
trees  whose  butts  are  anchored  to  piles  in  the  river  bed,  thus 
artificially  creating  a  structure  similar  to  the  drift  which  accumu- 
lates against  a  permeable  dike.  The  branches  of  the  trees  check 
the  flow  of  water  and  induce  a  deposit  of  sediment,  but  they  are 
sufficiently  flexible  to  permit  the  passage  of  ice  and  drift  over  them, 
without  destroying  the  structure.  There  is  also  not  as  great  a 
scour  at  the  outer  end  as  in  ordinary  dike  construction. 

Retards  have  been  efficient  as  substitutes  for  bank  revetment, 
particularly  when  the  river  has  not  been  regulated  systematically 
and  detached  protection  is  required  for  bridges  or  landings.  They 
adjust  themselves  to  changes  in  the  curvature  of  the  river  cur- 
rent more  readily  than  mattresses  which  have  been  sunk  several 
years.  At  localities  where  trees  are  of  little  value  and  stone 
is  expensive,  they  are  cheaper  than  the  ordinary  mattress  con- 
struction. 

When  the  amount  of  material  carried  in  suspension  is  small,  as 
on  the  upper  Mississippi  River,  the  permeable  dike  is  not  as  suc- 
cessful. Not  only  is  there  danger  of  its  destruction  by  drift  and 
ice,  but  the  portions  of  the  piles  above  low  water,  not  being  pro- 
tected by  sand  deposits,  become  alternately  wet  and  dry,  and 
rapidly  decay.  On  this  river  (1)  spur-dikes  made  of  brush  fas- 
cines and  covered  with  stone  to  hold  them  in  place  have  been 
employed  extensively.  These  dikes  are  protected  from  being  un- 
dermined by  the  water  flowing  over  their  crests  by  extending 
the  lower  layer  of  brush  about  ten  feet  below  the  dam  proper, 


58  RIVERS  AND   HARBORS 

while  the  sand  which  settles  in  them  affords  a  partial  protection 
from  settlement  due  to  decay.  On  many  rivers  of  the  United 
States,  the  dikes  are  constructed  of  rip-rap  or  of  gravel  covered 
with  rip-rap. 

On  the  Rhine,  prior  to  1880,  gabions  made  of  woven  brush  arid 
filled  with  gravel  were  employed  extensively  in  dike  construction 
to  the  level  of  low  water,  the  upper  portion  of  the  dike  being  com- 
posed of  gravel  covered  with  rip-rap.  Since  that  date,  the  chan- 
nel has  been  deepened  extensively  by  dredging,  and  the  body  of 
the  dikes  is  now  generally  composed  of  the  gravel  obtained 
from  the  dredging  and  is  protected  from  scour  by  rip-rap. 
The  downstream  faces  of  such  dikes  are  steep  and  are  protected 
by  rip-rap.  The  upstream  faces  have  gentle  slopes,  about  1  to 
4,  and  they  are  left  unprotected  when  the  current  is  not  too  strong. 

Since  dredging  has  been  introduced  as  an  auxiliary  to  the  im- 
provement of  the  upper  Mississippi  River,  there  is  also  a  tendency 
to  construct  dikes  of  the  material  dredged  from  that  river,  and  to 
cover  them  with  mattress  and  stone  revetments  similar  to  those 
used  in  bank  protection. 

Considerable  study,  particularly  by  German  authors,  has  been 
given  to  the  subject  of  the  proper  distances  between  spur-dikes, 
and  their  proper  inclination  to  the  direction  of  the  river  flow. 
The  early  German  practice  on  the  Rhine  was  to  construct  the 
dikes  with  a  considerable  inclination  downstream.  Such  dikes  in- 
duced a  scour  along  them  which  caused  considerable  settlement, 
and  such  a  caving  of  the  banks  between  them  that  it  was  necessary 
to  connect  the  outer  end  of  one  dike  with  the  inner  end  of  the  one 
next  below  it.  This  produced  the  triangle  werk  (2),  which  was 
employed  to  a  considerable  extent  on  the  German  Rhine. 

As  a  result  of  many  years  of  investigation,  the  practice  on 
German  rivers,  as  described  by  Schlichting  (3),  is  to  give  dikes 
an  inclination  upstream  of  105°  to  110°  on  straight  reaches, 
100°  to  102.5°  on  concave  banks,  and  90°  to  100°  on  convex  banks. 
They  are  located  so  that  their  axes  intersect  in  the  middle  of  the 
channel.  They  are  spaced  at  5/7  of  the  channel  width  in  straight 
reaches,  at  half  the  channel  width  on  concave  banks,  and  at  the 
full  width  on  convex  banks. 

The  French  and  Italian  practice  is  also  to  give  spur-dikes  a  slight 
inclination  upstream,  while  in  the  United  States  they  are  approxi- 
mately perpendicular  to  the  river  currents,  though  examples  of  dikes 


DIKE   CONSTRUCTION  AND   BANK   PROTECTION        59 

can  be  found  that  are  strongly  inclined  downstream.  This  is  true 
particularly  in  work  done  by  railroads  to  protect  their  tracks 
from  caving  where  they  are  constructed  along  a  river  bank.  A 
spur-dike  perpendicular  to  the  current  produces  the  maximum 
contraction  with  the  least  expenditure  of  material.  European 
spur-dikes  are  usually  given  an  excess  of  rip-rap  at  their  outer 
ends  for  the  purpose  of  rilling  the  hole  scoured  by  the  current 
passing  around  them.  The  proper  inclination  to  reduce  the  di- 
mensions of  this  scour  is  of  more  importance  in  Europe  than  in 
the  United  States,  where  the  scour  is  prevented  by  sinking  a  mat 
at  the  end  of  the  dike  similar  to  that  used  in  bank  protection. 

When  it  is  considered  necessary  to  construct  spur-dikes  on  the 
concave  bank  of  a  river,  the  permeable  dike  has  a  great  advantage 
over  one  that  is  not  permeable,  provided  the  river  carries  a  large 
amount  of  material  in  suspension.  In  the  discussion  of  the  laws 
governing  the  flow  of  water,  attention  was  invited  to  the  eddy 
action  produced  below  it  by  an  obstruction  extending  toward  the 
channel  from  the  bank  of  a  river.  This  eddy  not  only  induces  a 
deposit  behind  the  dike,  of  material  that  is  being  rolled  along  the 
river  bed,  but  also  produces  a  caving  of  the  bank  a  short  distance 
further  downstream.  If  a  permeable  dike  is  constructed,  the  flow 
along  the  river  bank  is  only  partially  checked,  and  therefore  the 
eddy  action  is  greatly  reduced.  The  fill  behind  the  dike,  instead  of 
being  entirely  due  to  material  rolled  along  the  bed,  is  caused  prin- 
cipally by  the  deposit  of  material  in  suspension;  and  the  point  of 
attack  of  the  eddy  current  on  the  river  bank  is  moved  such  a  dis- 
tance downstream  that  a  second  spur-dike  can  be  constructed 
economically  to  prevent  its  action.  By  means  of  permeable 
spur-dikes  it  is  thus  possible  to  build  up  by  deposits  a  new  bank 
of  any  curvature  desired  at  a  cost  less  than  that  of  the  dikes 
which  are  employed  in  Europe  to  connect  the  curved  longitudinal 
dike  to  the  bank.  The  cost  of  revetting  the  new  bank  line  thus 
formed  is  much  less  than  the  cost  of  constructing  a  longitudinal 
dike. 

Spur-dikes  on  a  concave  bank  are  much  more  liable  to  be  de- 
stroyed by  floating  debris  and  ice,  however,  than  are  those  on  the 
convex  bank,  since  the  helicoidal  flow  of  water  in  a  bend  carries 
material  floating  on  the  surface  of  the  river  toward  the  concave 
bank.  The  downward  flow  of  the  water  along  the  newly  built-up 
bank  gives  it  a  steep  slope  towards  the  river,  so  that  the  ends  of 


60  RIVERS  AND  HARBORS 

the  dikes  project  considerably  above  the  river  bed.  After  the 
new  bank  has  been  formed,  it  is  therefore  necessary  to  cut  down 
the  outer  ends  of  the  spur-dikes  to  the  slope  of  the  new  bank,  and 
to  protect  the  bank  with  a  revetment,  or  the  work  will  be  gradually 
destroyed. 

When  a  longitudinal  dike  is  not  constructed  in  a  bend,  the 
method  of  bank  protection  at  present  usually  adopted  on  European 
rivers  is  a  covering  of  rip-rap.  "Quarry  stone  is  deposited  on  the 
trace*  of  the  work  and  there  takes  its  natural  slope.  On  rivers 
with  a  movable  bed,  the  first  rip-rap  sinks,  the  mass  is  reenforced, 
the  soil  is  consolidated,  and  after  a  greater  or  less  length  of  time, 
according  to  the  mobility  of  the  bed  and  the  swiftness  of  the  cur- 
rent, stability  is  acquired,  and  the  slope  then  assumes  the  form 
of  a  concave  curve"  (4). 

Earlier  French  authorities  (5)  for  reasons  of  economy  recom- 
mended the  substitution  of  gabions  filled  with  gravel  for  a  portion 
of  the  rip-rap,  a  method  of  bank  protection  which  also  has  been 
employed  extensively  by  the  Italians  on  the  Po  River.  The  rip- 
rap is  generally  given  a  slope  below  low  water  of  1  vertical  to  2 
horizontal,  though  there  are  examples  on  the  Po  of  slopes  of  1  to 
1 J.  The  thalweg  depths  in  bends  thus  protected  are  usually  large. 

In  the  first  attempts  to  protect  the  banks  of  the  Rhine  (2),  the 
Germans  employed  masses  of  brush  made  into  fascines  and  sunk 
along  the  bank  to  be  protected,  which  fascine  mass  had  a  width 
approximately  equal  to  the  river  depth.  This  method  of  bank 
protection  was  called  bleeswerk.  Following  the  practice  in  Hol- 
land, the  slope  of  the  protection  facing  the  river  was  first  made 
vertical,  but  excessive,  scour  resulting  therefrom,  the  bleeswerk 
was  given  a  slope  of  1  to  1  by  a  royal  decree  of  1744.  The  under- 
mining and  destruction  of  the  work  still  continued,  however,  as 
illustrated  by  the  attempts  to  protect  the  bank  at  the  city  of 
Wesel.  There  was  substituted  for  bleeswerk  first  the  spur-dike 
inclining  downstream,  and  then  the  triangle  werk.  These  meth- 
ods also  gave  such  unsatisfactory  results  that  for  a  time  all  methods 
of  bank  protection  were  abandoned,  and  recourse  was  had  either  to 
diverting  the  river  from  important  localities  threatened  with  de- 
struction by  caving  banks,  by  cutting  an  auxiliary  channel  through 
the  neck  of  land  opposite  them,  as  at  Wesel,  or  by  moving  the 
smaller  villages  further  inland.  Much  of  the  river  straightening 
of  the  lower  Rhine  has  resulted  from  this  cause. 


DIKE    CONSTRUCTION   AND    BANK   PROTECTION        61 

Later  Herr  Nobling,  when  in  charge  of  the  improvement  of 
the  Rhine,  employed  a  submerged  spur-dike  called  a  Nobling, 
which  has  been  successfully  used  in  bank  protection,  particularly 
on  straight  reaches.  The  Noblings  were  originally  constructed  of 
gravel,  protected  from  scour  by  gabions  filled  with  gravel,  and 
more  recently  of  gravel  covered  with  rip-rap.  They  extended 
into  the  river  from  the  mid-stage  on  the  bank  with  a  slope  of  about 
1  to  4.  The  upper  bank  was  given  a  slope  of  1  to  2  and  was  paved 
with  rip-rap  stone. 

In  concave  bends  caving  occurs  between  the  Noblings,  and  it 
has  been  necessary  to  protect  the  bank  between  them,  for  which 
purpose  the  material  obtained  by  dredging  the  river  has  been  em- 
ployed extensively,  with  a  covering  of  rip-rap.  A  similar  type 
of  structure  has  been  constructed  on  the  Mississippi  river  to  pro- 
tect the  bends  in  front  of  the  cities  of  New  Orleans  and  Greenville. 
At  New  Orleans  it  has  been  necessary  to  supplement  the  dikes 
with  a  mattress  revetment  between  them.  At  Greenville,  the 
floods  of  1890,  '91,  '92,  and  '93  caused  such  a  caving  between  the 
dikes  that  several  were  undermined  and  destroyed,  and  have  been 
replaced  by  mattress  revetments. 

In  the  United  States,  while  the  rip-rap  of  stone  has  been  em- 
ployed occasionally  for  bank  protection,  the  great  depth  found 
in  the  bends  of  the  western  rivers  and  the  scour  that  is  caused  by 
the  revetment  has  rendered  this  method  very  expensive.  Except 
for  small  streams,  there  usually  is  substituted  a  subaqueous  mat 
of  brush  with  an  upper  bank  protection  either  of  rip-rap,  of  stone 
more  carefully  laid  as  a  pavement,  or  of  concrete.  There  are 
great  variations  in  the  method  of  constructing  the  mat,  due  prin- 
cipally to  the  character  of  the  brush  that  is  found  in  the  vicinity 
of  the  work.  In  the  Mississippi  River,  between  St.  Paul  and  the 
mouth  of  the  Missouri  (1),  the  brush  is  made  into  fascines  about 
twelve  inches  in  diameter,  which  are  bound  together  by  binding 
poles  and  laid  parallel  to  the  bank.  On  the  Mississippi,  between 
Cairo  and  Vicksburg  (6),  the  fascines  have  a  diameter  of  about 
sixteen  inches  and  are  laid  perpendicular  to  the  bank.  Between 
St.  Louis  and  Cairo  (7)  the  brush  is  woven  through  the  binding 
poles.  On  the  Missouri  River  (8) ,  a  much  smaller  willow  brush  is 
employed  than  on  the  Mississippi,  and  the  method  of  weaving 
gives  to  the  upper  surface  of  the  mattress  the  appearance  of  a 
woven  basket. 


62  RIVERS  AND   HARBORS 

Where  brush  is  difficult  to  obtain,  a  mattress  of  boards  has  been 
employed  (9).  The  boards  are  of  the  class  commercially  known 
as  culls,  and  are  from  4  to  8  inches  in  width  and  one  inch  in  thick- 
ness. They  are  woven  through  the  binding  poles  similarly  to 
brush.  This  form  of  revetment  has  been  successful  on  the  upper 
and  middle  Mississippi  but  has  quite  generally  failed  on  the  Red 
River.  It  is  believed  that  this  failure  is  due  to  faulty  construction. 
Recently  extensive  experiments  have  been  made  on  the  lower 
Mississippi  in  utilizing  concrete  as  a  subaqueous  bank  protection 
(10).  Both  thin  reenforced  concrete  slabs  and  reenforced  con- 
crete blocks  have  been  used. 

The  essential  conditions  that  a  mattress  must  fulfil  are  that  it 
should  have  sufficient  thickness  to  protect  the  soil  on  which  it  is 
laid  from  the  action  of  river  currents,  and  sufficient  pliability  to 
adapt  itself  to  the  irregularity  of  the  ground.  The  various  types 
of  mattresses  conform  to  these  requirements.  There  are,  how- 
ever, certain  precautions  which  must  be  observed  in  mattress 
construction,  or  serious  breaks  are  liable  to  occur  in  the  revetment. 

A  caving  bank  ordinarily  assumes  under  water  the  natural  slope 
of  the  material  of  which  it  is  composed,  due  to  the  constant  supply 
of  such  material  by  the  caving.  When  the  revetment  is  con- 
structed, this  supply  ceases  along  the  part  protected.  Since  the 
eroding  force  still  continues  to  act,  there  is  a  tendency  to  deepen 
the  portion  of  the  river  beyond  the  revetment.  Hence  the  scour 
at  the  outer  edge  may  be  very  great  if  the  mattress  does  not  ex- 
tend beyond  the  thalweg.  This  produces  a  surface  with  a  steeper 
slope  than  the  saturated  material  of  which  it  is  composed  can 
maintain,  and  large  masses  will  slide  into  the  river,  carrying  the 
revetment  with  it.  The  mattress  therefore  must  extend  a  suf- 
ficient distance  beyond  the  thalweg  to  protect  the  slope  and  to 
prevent  it  from  attaining  an  unstable  inclination. 

A  sufficient  amount  of  stone  must  be  placed  also  on  the  outer 
edge  of  the  mattress  to  force  it  to  sink  into  the  deepened  channel 
as  fast  as  the  scour  occurs.  If  not,  the  mat  will  be  undermined. 
When  green  willow  brush  is  first  cut,  it  is  very  pliable,  but  after 
several  seasons  of  exposure  to  water,  it  becomes  quite  brittle. 
A  change  in  the  direction  of  the  currents  may  produce  a  deepening 
of  the  channel  by  increasing  the  scouring  power  several  years  after 
the  mat  has  been  laid,  and  unless  an  excess  of  stone  is  added 
originally,  the  old  mat  cannot  follow  the  change.  A  stiff  mat  pro- 


DIKE   CONSTRUCTION  AND   BANK   PROTECTION        63 

jecting  into  the  river  currents  and  unsupported  by  the  river  bank 
is  liable  to  destruction  by  the  vibrations  caused  by  whirls  and 
eddies. 

If  the  caving  bank  is  not  homogeneous,  but  is  composed  of  layers 
possessing  different  resistances  to  erosion,  the  slope  that  it  will 
assume  during  caving  on  a  rising  river  will  not  be  regular,  but  will 
be  steeper  in  those  portions  which  most  resist  erosive  action.  Such 
a  bank  may  be  in  an  unstable  equilibrium  and  as  the  river  falls, 
a  slide  may  occur  where  the  slopes  are  too  steep  to  sustain  the 
pressure  exerted  by  the  weight  of  the  soil  above.  By  giving  a 
gentle  slope  to  the  portion  of  the  bank  above  low  water,  this 
tendency  may  be  minimized,  but  even  with  slopes  as  gentle  as  1 
to  4,  such  slides  occur,  carrying  the  mattress  with  them  and  causing 
a  break  in  the  revetment  near  the  low-water  line.  On  the  portion 
of  the  Mississippi  River  above  Cairo  such  breaks  are  comparatively 
small  and  can  be  repaired  readily  during  the  next  low-water 
season;  but  on  the  lower  river,  on  account  of  the  great  depth  in 
the  bends,  they  may  attain  large  dimensions,  and  may  seriously 
damage  the  revetment. 

Another  cause  of  failure  of  mattresses  is  the  passage  through 
them  of  the  material  composing  the  bank  in  sufficient  quantities 
to  cause  the  bank  to  settle.  This  action  is  rarely  due  to  the  scour 
of  river  currents  through  the  mat,  but  arises  during  low  stages 
from  a  flow  of  water  from  the  bank  into  the  river,  carrying  a  large 
amount  of  sand  with  it.  The  source  of  the  water  supply  is  usually 
a  neighboring  pond  and  the  bank  must  contain  a  layer  of  sand 
readily  acted  upon  by  the  flow  through  it.  This  danger  usually 
can  be  removed  by  the  proper  drainage  of  the  land  in  the  vicinity 
of  the  revetment.  The  borrow  pits  for  levee  construction  fre- 
quently become  stagnant  pools  of  this  character. 

The  most  serious  breaks  in  revetments,  however,  have  resulted 
from  failure  to  complete  the  improvement  of  a  bend  by  the  con- 
struction of  the  necessary  spur-dikes  on  the  convex  bank  after 
the  concave  bank  has  been  protected.  Attention  has  been 
invited  to  the  tendency  of  the  river  currents  to  assume  different 
radii  of  curvature  at  different  stages.  It  has  happened  not  in- 
frequently that  a  revetment  has  been  constructed  and  the  con- 
struction of  the  spur-dikes  opposite  it  has  been  deferred  either 
from  lack  of  funds  or  from  a  desire  to  protect  some  other  threat- 
ened locality.  A  flood  then  occurring  scours  a  channel  across  the 


64  RIVERS  AND   HARBORS 

projecting  point  and  the  river  currents  impinge  almost  perpen- 
dicularly on  some  portion  of  the  revetment.  A  sudden  change  of 
direction  with  a  short  radius  of  curvature  is  created  which  scours 
a  deep  hole  at  the  foot  of  the  revetment,  and  the  revetment  slides 
into  this.  The  channel  along  the  revetment  above  the  point  of 
impact  frequently  is  filled  with  sediment  deposited  by  eddy  action 
and  a  serious  disturbance  of  the  river's  regimen  results.  A  cut-off 
will  cause  disastrous  caving  in  a  similar  manner  by  changing  the 
river  slopes  and  thus  affecting  the  radius  of  curvature  of  the  flow. 

The  mattress  usually  is  carried  up  the  bank  to  the  elevation  of 
the  water  surface  at  the  time  of  construction,  but  the  portion 
above  low  water  is  subject  to  decay  and  should  be  covered 
by  a  rip-rap  of  stone  to  insure  the  permanence  of  the  work. 

The  upper  bank  is  graded  to  the  prescribed  slope  (from  1  on  2 
to  1  on  4)  and  either  is  paved  with  stone  laid  so  as  to  give  a  thick- 
ness of  about  one  foot,  or  is  covered  with  about  four  inches  of 
concrete  (11).  The  grading  frequently  is  done  by  manual  labor  or 
by  scrapers.  On  the  high  banks  of  the  lower  Mississippi,  hy- 
draulic grading  usually  is  employed. 

The  use  of  stone  or  concrete  for  the  upper  bank  pavement  is 
dependent  on  the  proximity  of  stone  or  gravel  to  the  work.  Where 
stone  can  be  quarried  in  the  vicinity,  it  is  more  economical  than 
concrete,  but  if  it  has  to  be  transported  long  distances,  concrete 
may  be  cheaper.  Concrete  offers  a  greater  resistance  to  the  action 
of  ice  than  a  stone  pavement,  because  the  ice  freezes  around  the 
individual  pieces  of  stone  and  is  liable  to  remove  them  from  the 
bank  when  it  starts  to  flow  downstream  in  the  spring.  Floating 
ice  will  also  more  readily  slide  over  a  concrete  revetment,  while  it 
tends  to  dislodge  fragments  of  a  stone  pavement. 

Both  types  of  revetment  require  protection  from  rain  water 
flowing  down  the  slopes  and  washing  out  cavities  under  them 
during  low  stages  of  the  river.  A  ditch  at  the  top  of  the  bank, 
with  occasional  properly  paved  drains  down  the  slope,  will  eliminate 
this  danger.  Wave  action  from  severe  wind  storms  is  more  de- 
structive to  a  bank  paved  with  stone  than  to  one  covered  with 
concrete. 

While  the  brush  mat  affords  a  more  economical  method  of  bank 
protection  than  a  deposit  of  rip-rap,  it  is  usually  too  expensive  a 
method  to  be  employed  profitably  for  the  protection  of  agricul- 
tural lands  except  when  the  revetment  is  an  adjunct  to  the  im- 


DIKE    CONSTRUCTION   AND    BANK   PROTECTION        65 

provement  of  the  river  channel  for  navigation,  and  numerous  sub- 
stitutes have  been  suggested,  many  of  which  have  been  patented. 
There  is  a  marked  tendency  in  the  United  States  to  rediscover 
methods  of  bank  protection  which  have  been  tested  in  Europe  for 
many  years  and  have  been  found  defective.  Thus,  the  blees- 
werk  abandoned  on  the  Rhine  in  1796  has  its  advocates  on  the 
upper  Missouri  River.  Fine  examples  of  dikes  extending  down- 
stream, rejected  by  the  Germans  over  one  hundred  years  ago,  are 
to  be  found  in  the  protection  of  railroad  tracks  on  the  Kaw  River 
in  Kansas;  and  the  triangle  werk  which  failed  at  Wesel  has  been 
reproduced  on  some  of  our  western  rivers.  The  suspension  of  the 
trunks  of  trees  along  a  caving  bank  is  but  a  weak  imitation  of  the 
tunnages  ordinaires,  described  by  Defontaine  (5)  in  1830,  as  then 
employed  by  the  French  along  the  Rhine  in  Alsace.  The  Neal 
dikes  are  based  on  the  principles  of  the  Noblings,  but  instead  of 
filling  the  gabions  with  gravel  on  land  and  sinking  them  in  place, 
a  cellular  structure  of  brush  and  poles  is  constructed  in  the  river 
bed  and  allowed  to  fill  with  sediment,  a  method  of  construction 
used  by  the  Italians  in  the  Po  prior  to  1682.  They  have  the  same 
defect  as  the  Noblings  of  requiring  bank  protection  between 
them  in  sharp  bends.  However,  where  the  river  bank  has  been 
scoured  to  an  irregular  slope  and  there  is  danger  of  sliding  if  pro- 
tection is  afforded  by  a  simple  revetment,  a  cellular  structure  can 
be  constructed  on  the  revetment  which  will  rapidly  fill  with  sedi- 
ment and  a  stable  slope  will  thus  be  formed.  This  method  has 
recently  been  employed  successfully  for  the  protection  of  the 
abutments  of  the  Iron  Mountain  railroad  bridge  across  the  Ar- 
kansas River  near  Watson. 

The  Fuller  timber  dikes  inclining  upstream,  unsuccessfully 
employed  on  the  Red  River,  have  their  prototypes  in  the  pennelli 
(12)  of  the  Po  River,  constructed  in  the  seventeenth  century  to  pro- 
tect Cremona.  The  Brownlow  weed  and  other  devices  which  rely 
on  the  floating  branches  of  trees  to  deposit  sediment  are  similar  to 
the  Epi  de  Branchage  employed  on  the  Midouze  River  in  France, 
and  described  by  M.  Malezieux  in  his  Cours  de  Navigation  Int'e- 
rieure  in  1876,  but  instead  of  being  anchored  to  the  river  bed,  the 
branches  were  attached  to  piles. 

Attention  has  been  called  to  the  use  of  permeable  spur-dikes 
to  cause  deposits  on  the  concave  bank  of  a  river.  Permeable 
longitudinal  dikes  also  have  been  employed  for  the  same  purpose, 


66  RIVERS  AND  HARBORS 

particularly  on  the  Missouri  in  the  vicinity  of  St.  Joseph.  Such 
dikes  induce  a  rapid  fill,  and  floating  debris  and  ice  have  no  op- 
portunity to  lodge  along  them,  as  is  the  case  with  spur-dikes. 
River  currents,  however,  induce  a  scour,  which  renders  it  necessary 
to  protect  the  river  bed  with  wide  mats;  and  the  fill  which  accumu- 
lates exerts  a  lateral  pressure  so  that  the  dike  has  to  act  as  a  re- 
taining wall,  for  which  purpose  it  is  not  well  adapted.  During 
periods  of  low  water,  the  piles  composing  it  are  subject  to  decay, 
and  during  floods  to  an  erosive  action  by  the  movement  of  debris 
and  ice  along  them.  After  a  limited  life  it  is  usually  necessary  to 
replace  the  structures  with  some  form  of  revetment. 


CHAPTER  VIII 
RIVER  IMPROVEMENT  BY  CANALIZATION 

Wherever  a  river's  slope  is  locally  diminished,  the  depth  over 
its  bars  is  increased.  By  building  dams  across  the  river  channel, 
pools  are  created  above  the  dams  which  have  very  gentle  slopes 
during  low  water,  the  greatest  change  in  the  elevation  of  the  water 
surface  occurring  at  the  dam.  This  principle  has  been  employed 
extensively  in  improving  the  navigation  of  rivers,  and  is  particularly 
successful  when  the  low-water  discharge  and  the  amount  of  sedi- 
ment transported  are  small. 

The  material  moved  by  such  rivers  during  low  stages  is  insig- 
nificant. During  high  stages  it  tends  to  form  bars  immediately 
below  the  dams,  through  which  channels  can  be  maintained  readily 
by  dredging.  As  the  sediment  carried  increases,  the  amount  of 
dredging  required  may  become  so  large  as  to  make  the  cost  of 
maintenance  excessive.  By  substituting  for  fixed  dams  some  of 
the  numerous  movable  types  which  have  been  invented,  this  item 
of  maintenance  can  be  reduced  materially. 

A  movable  dam  is  one  in  which  the  portions  that  obstruct  the 
river's  flow  may,  when  it  is  desired,  be  lowered  to  an  elevation 
previously  determined  above  the  river  bed,  or  removed  from  the 
river  channel.  By  properly  operating  such  dams  when  the  amount 
of  sediment  moving  in  a  river  increases,  its  velocity  may  be  so 
regulated  that  material  in  suspension  does  not  tend  to  deposit, 
and  the  material  rolled  along  the  bed  conforms  to  the  laws  govern- 
ing it  in  an  unobstructed  river.  During  high  stages  the  unob- 
structed portions  of  the  river  bed  become  navigable  passes.  It  is 
then  unnecessary  for  vessels  to  pass  through  locks  to  overcome 
the  difference  of  level  that  exists  at  the  dam  during  low  water. 
They  also  reduce  the  gage-heights  which  fixed  dams  cause  in  the 
flood  discharge,  which  is  an  important  consideration  in  thickly 
populated  valleys. 

Dams  have  been  divided  into  two  classes,  fixed  and  movable. 
Movable  dams  are  divided  into  certain  types  dependent  on  their 
moving  parts.  In  recent  construction,  it  is  exceptional  that  a 
dam  is  constructed  of  a  single  type.  Even  fixed  dams  have 

67 


68  RIVERS  AND   HARBORS 

sluices  controlled  by  movable  gates,  and  a  movable  dam  has  fre- 
quently one  type  of  closure  for  its  navigable  pass,  and  possibly 
two  types  of  gates  for  its  weirs.  Occasionally  a  dam  is  of  the  fixed 
type  to  a  certain  elevation  and  is  surmounted  by  a  movable  crest 
which  is  employed  not  only  to  regulate  flood  heights  but  also  to 
afford  a  navigable  pass  during  high  stages  when  the  fixed  portion 
of  the  dam  is  drowned  out.  On  a  rising  river,  the  stage  increases 
more  rapidly  at  the  foot  of  a  dam  than  on  its  crest,  but  the  rate 
is  variable  and  depends  on  the  stage,  on  the  slope  of  the  river,  and 
on  the  form  of  the  valley  below  the  dam.  At  some  stages,  a  rise 
of  one  foot  on  the  crest  of  the  dam  is  frequently  accompanied  by 
a  rise  of  from  two  to  three  feet  at  its  foot.  Hence,  at  certain 
stages,  the  slope  over  the  dam  becomes  so  slight  that  boats  can 
navigate  the  main  river  channel,  provided  there  is  a  sufficient 
depth  over  a  portion  of  the  crest  of  the  dam,  and  boats  can  thus 
avoid  the  delays  incident  to  the  passage  through  a  lock. 

Movable  dams  were  first  practically  used  for  improving  navi- 
gation by  the  French.  In  their  early  construction,  Poiree  (1) 
needles  were  employed  extensively,  but  the  inventive  genius  of  the 
French  and  of  other  nations  has  led  to  a  confusing  multiplicity 
of  types,  even  on  the  same  river  and  frequently  on  the  same  dam. 
While  there  has  been  considerable  discussion  of  the  advantages 
and  disadvantages  of  the  different  types,  it  is  not  always  evident 
that  even  local  conditions  justify  many  of  the  variations. 

The  Poiree  needles  are  of  timber,  usually  of  rectangular  cross- 
section.  In  the  earlier  designs,  they  were  of  such  weight  that 
they  could  be  placed  readily  in  their  proper  position  by  hand. 
When  placed  in  the  dam,  their  lower  ends  rest  against  a  sill  of 
timber  or  preferably  of  concrete,  and  their  upper  ends  against  a 
bar  which  is  supported  by  trestles  hinged  to  the  base  of  the  dam 
so  that  they  can  be  lowered  behind  the  sill  when  the  needles  have 
been  removed.  The  free  ends  of  the  trestles  are  connected  by  a 
chain.  The  process  of  raising  the  dam  consists  in  pulling  on  the 
chain  till  the  trestles  are  in  an  upright  position.  The  bar  is  then 
placed  against  the  trestles,  and  the  needles  are  placed  side  by 
side,  supported  by  the  sill  and  the  bar.  A  foot-bridge  is  con- 
structed on  the  trestles  to  enable  these  operations  to  be  performed. 

As  the  needles  were  made  larger,  difficulty  was  experienced  in 
removing  them  and  the  supporting  bars  were  replaced  by  escape 
bars  so  arranged  that  the  bar  of  any  pair  of  trestles  could  be 


RIVER   IMPROVEMENT   BY   CANALIZATION  69 

tripped.  This  released  the  needles  between  two  trestles,  and  they 
were  allowed  to  float  down  the  river,  held  together  by  a  rope  so 
that  they  could  be  recovered.  A  hook  was  also  fastened  to  the 
upper  end  of  the  needle,  so  that  power  could  be  applied  through 
a  winch. 

By  substituting  stop-planks  for  the  needles,  the  Boule  gate-dam 
was  evolved  (1).  The  ends  of  the  stop-planks  rest  against  the 
faces  of  the  trestles,  and  slide  along  them.  The  sliding  friction 
limits  the  size  of  the  gate  which  can  be  handled  readily,  but  by 
causing  it  to  move  on  rollers  this  friction  can  be  largely  reduced, 
and  the  size  of  the  gate  increased.  By  inserting  between  the  gate 
and  the  support  a  chain  of  rollers,  the  Stoney  Gate  is  created. 
This  form  of  gate  can  be  manipulated  with  a  small  expenditure 
of  power,  so  that  gates  of  large  size  can  be  constructed. 

When  weirs  are  regulated  by  large  gates,  masonry  piers  are 
frequently  substituted  for  the  metallic  trestles,  and  a  masonry 
structure  is  substituted  for  the  wooden  foot-bridge,  so  that  the 
sluice-ways  become  openings  in  the  face  of  the  dams.  Over  the 
navigable  pass  both  the  trestles  and  gates  may  be  supported  by  a 
steel  bridge  of  sufficient  clearance  to  permit  boats  navigating  the 
stream  to  pass  under  it  when  the  movable  parts  are  raised.  Such 
bridges  may  be  utilized  as  road-bridges,  as  are  some  of  those 
constructed  over  the  Mohawk  River  on  the  New  York  Barge 
Canal.  The  Emergency  Dams  of  the  Panama  Canal  (2),  and  those 
at  Sault  Ste.  Marie,  consist  of  swing-bridges  which  are  swung 
across  the  channels  and  from  which  the  trestles  and  gates  are 
lowered  when  required. 

For  the  gate  there  has  been  substituted  in  some  cases  the  Camere 
curtain  (3),  which  "  consists  of  narrow  horizontal  strips  of  wood 
hinged  together,  and  capable  of  being  rolled  up  by  an  endless 
chain  which  passes  round  them,  each  curtain  reaching  from  the 
surface  of  the  water  to  the  sill."  For  the  curtain  there  has  been 
recently  substituted  the  rolling  dam  (4) ,  which  consists  of  a  metallic 
cylinder  resting  on  masonry  supports.  When  not  in  use,  it  is 
rolled  up  an  inclined  plane  above  the  water  surface  by  powerful 
machinery. 

In  the  dams  described  above,  the  parts  which  prevent  the  flow 
of  the  water  are  removed  from  the  dam  during  floods.  In  a  large 
variety  of  types,  they  are  lowered  to  the  river  bed.  The  Thenard 
shutter  (1)  was  one  of  the  early  forms  of  movable  dams  adopting 


70  RIVERS  AND  HARBORS 

this  principle.  It  consists  of  a  shutter  hinged  to  the  base  and  held 
in  position  by  a  prop  resting  against  the  shoulder  of  a  casting, 
called  a  hurter.  It  was  soon  developed  into  the  Chanoine  wicket 
(1),  which  has  been  adopted  very  extensively  for  the  navigable 
pass  both  in  France  and  in  the  United  States.  In  this  type  of 
dam  the  shutter,  instead  of  rotating  about  its  lower  end,  is  hinged 
above  its  middle  third  to  a  frame  called  a  horse.  The  horse  also 
rotates  about  a  hinge  connecting  it  to  the  base,  and  is  held  in  po- 
sition by  a  prop  supported  in  a  hurter. 

The  Thenard  shutter  offered  such  a  resistance  to  the  water 
while  being  placed  in  position,  that  an  auxiliary  dam  of  counter 
shutters  had  to  be  erected  before  the  props  could  be  engaged  in 
the  hurter.  The  horse  of  the  Chanoine  wicket,  however,  can  be 
raised  and  its  prop  placed  in  position  while  the  shutter  is  floating 
on  the  water's  surface;  and  after  the  horse  is  erected,  the  wicket 
can  be  swung  into  position  by  pressure  on  its  lower  extremity. 

Two  methods  are  employed  for  lowering  the  dam,  and  the 
hurters  have  to  be  adjusted  to  the  method  employed.  For 
narrow  passes,  a  tripping  bar  is  frequently  used.  It  consists  of  a 
flat  steel  bar  with  projecting  teeth  which  moves  in  a  groove  on 
the  hurters  close  to  the  props,  the  projecting  teeth  being  so 
arranged  as  to  press  successively  against  them.  The  tripping  bar 
is  manipulated  by  gearing  in  the  piers  or  abutments,  and  the 
teeth  force  the  props  from  the  shoulder  into  a  groove  beside  it, 
along  which  the  prop  slides  until  the  horse  and  wicket  assume 
a  horizontal  position. 

Another  method  of  lowering  the  dam  is  to  pull  the  wicket  for- 
ward until  the  prop  is  released  from  the  shoulder  of  the  hurter, 
and  the  groove  is  so  constructed  as  to  cause  the  hurter  to  slide 
into  it  in  this  advanced  position.  Such  an  arrangement  is  called 
a  Pasqueau  hurter.  In  addition  to  the  groove  which  guides  the 
prop  while  the  wicket  is  falling,  there  is  a  second  groove  which 
guides  it  to  its  proper  position  on  the  shoulder  when  the  horse 
is  raised.  The  wickets  can  be  raised  and  lowered  from  a  foot- 
bridge or  from  a  maneuvering  boat. 

The  Girard  shutter  (1)  is  a  modification  of  the  Thenard  shutter, 
in  which  the  prop  is  connected  to  an  hydraulic  piston  which  forces 
the  shutter  into  its  proper  position  and  holds  it  there  by  means 
of  water  pressure  against  the  piston.  By  reducing  the  water 
pressure  the  shutter  is  lowered, 


RIVER   IMPROVEMENT   BY   CANALIZATION  71 

In  the  Desfontaines  drum-wicket  (1),  the  shutter  which  closes 
the  weir  is  rigidly  connected  to  a  second  shutter  operating  in  a 
chamber  called  the  drum,  which  is  constructed  in  the  masonry  of 
the  dam.  By  applying  water  pressure  to  the  proper  side  of  the 
leaf  inclosed  in  the  drum,  the  upper  shutter  is  raised  and  held  in 
position,  and  by  removing  the  pressure  the  dam  is  lowered.  By 
locating  the  drum  on  the  downstream  side  of  the  shutter,  and 
placing  in  it  a  floating  pontoon  which  rises  and  falls  by  hydraulic 
pressure,  the  shutter  can  be  raised  and  lowered.  The  Krantz 
wicket  (1)  with  pontoon  works  on  this  principle,  the  pontoon  being 
pivoted  to  the  dam  along  its  lower  edge  and  the  shutter  pivoted 
on  the  pontoon  and  sliding  along  a  masonry  face  as  it  rises  and 
falls. 

In  the  original  Brunot  gate  (1),  the  shutter  is  connected  to  the 
dam  by  hinges,  and  the  pontoon  slides  along  it  when  being  ma- 
neuvered. In  the  modified  Brunot  gate,  the  caisson  and  shutter 
rotate  around  the  same  pivot,  and  the  shutter  becomes  the  deck 
of  the  pontoon. 

The  Chittenden  drum-wicket  (3)  works  on  the  .same  principle, 
but  the  shutter  is  supported  by  a  frame  which  forms  the  sector 
of  a  water-tight  cylinder,  and  the  bottom  of  the  pontoon  is  re- 
moved. 

The  Taintor  gate  (3)  is  similar  in  form  to  the  Chittenden  drum- 
wicket,  but  the  axis  of  rotation  is  placed  above  the  water  surface, 
and  the  stresses  are  reversed,  the  sector  of  the  cylinder  closing 
the  opening  instead  of  the  upper  radii.  The  gate  is  raised  to  per- 
mit a  flow  through  the  weir,  and  frictional  resistance  is  largely 
reduced  because  rotating  motion  is  substituted  for  sliding  motion. 

In  the  bear-trap  dams,  two  shutters  are  used  which  overlap 
one  another  when  the  dam  is  lowered.  Through  a  chamber  under 
the  leaves,  hydraulic  pressure  can  be  applied  which  will  cause 
them  to  rise.  In  the  first  dams  of  this  type  constructed  (10), 
both  shutters  were  hinged  to  the  foundation  of  the  dam  and  one 
leaf  slid  along  the  other  as  the  dam  rose.  In  the  Carro  dam  (1), 
the  downstream  leaf  is  hinged  to  the  dam,  while  the  upstream 
leaf  is  hinged  to  the  downstream  leaf  and  slides  along  the  crest 
of  the  weir  as  the  gate  is  maneuvered.  The  bear-trap  dam  of 
the  Chicago  Drainage  Canal  is  of  this  type,  except  that  the  up- 
stream leaf  slides  along  the  face  of  the  dam,  instead  of  along  its 
crest,  and  the  gate  is  balanced  by  counterweights. 


72  RIVERS  AND   HARBORS 

In  the  Lang  bear-trap  dam  (3),  the  upper  leaf  consists  of  two 
parts,  one  hinged  to  the  dam  and  the  other  to  the  lower  gate. 
These  parts  slide  on  one  another  as  the  gate  is  maneuvered.  In 
the  Parker  bear-trap  dam  (3),  the  two  parts  of  the  upper  gate  are 
also  hinged  together,  so  that  the  gate  folds  as  it  is  lowered. 

In  the  Thomas  A-frame  dam  (3),  the  shutter  is  rigidly  connected 
to  its  prop,  and  both  are  hinged  to  the  dam  so  that  they  can  be 
lowered  into  a  recess  on  the  masonry  along  the  crest  of  the  dam. 

There  are  numerous  minor  modifications  of  these  types.  An 
attempt  to  discuss  their  relative  merits  would  extend  this  book 
unduly  and  invade  the  domain  of  a  treatise  on  mechanical  engi- 
neering. The  Poiree  needles  and  Chanoine  wickets  with  Pasqueau 
hurters  are  the  types  usually  employed  in  the  United  States  for 
closing  navigable  passes;  the  original  bear-trap  dam  is  used  for 
closing  weirs  which  require  rapid  manipulation;  and  for  closing 
sluices  on  fixed  dams,  the  Stoney  and  Tain  tor  gates  usually  are  fav- 
ored on  account  of  the  small  frictional  resistance  in  operating  them. 

In  early  days  timber  was  extensively  employed  not  only  for 
the  movable  parts  of  dams,  but  also  for  fixed  dams  in  the  form 
of  timber  cribs  filled  with  stone.  At  the  present  time,  metal 
replaces  timber  as  far  as  practicable  in  the  movable  parts,  and 
the  fixed  dams  generally  are  made  of  concrete.  Gravity  types  of 
dams  also  have  been  replaced  in  some  cases  by  hollow  dams  of 
reinforced  concrete  or  of  steel.  They  are  sometimes  given  an  arched 
form,  with  their  stability  depending  on  transferring  the  stresses  to 
which  they  are  subjected  to  the  abutments.  In  a  gravity  dam,  the 
necessary  weight  given  it  to  prevent  overturning  usually  affords 
an  ample  coefficient  of  safety  against  sliding,  but  in  movable 
dams,  and  in  fixed  dams  for  which  reliance  is  placed  on  water 
pressure  to  hold  them  in  position,  a  special  study  of  this  source 
of  failure  is  necessary  in  connection  with  their  design. 

When  water  power  was  developed  from  the  water  wheel  and 
was  utilized  in  the  local  mill,  dams  of  low  elevation  prevailed, 
but  with  the  development  of  electric  power,  there  has  been  a 
tendency  to  build  higher  dams,  even  when  the  dams  are  adopted 
primarily  for  purposes  of  navigation.  As  a  result,  foundation  dif- 
ficulties have  increased  greatly.  Failures  due  to  sliding  have 
occurred  in  movable  dams  founded  on  rock,  and  they  have  been 
quite  numerous  where  reliance  has  been  placed  on  a  foundation 
of  gravel,  sand,  or  silt. 


RIVER   IMPROVEMENT   BY   CANALIZATION  73 

Homogeneous  impermeable  rock  makes  a  perfect  foundation  for 
a  structure,  but  it  is  rarely  found  in  river  beds.  Crevices  and 
fissures  usually  exist  which  permit  the  passage  of  considerable 
water  under  high  heads.  This  exerts  an  upward  pressure  on  the 
base  of  the  dam.  Little  reliance  should  be  placed  on  the  adhesion 
of  concrete  to  rock,  particularly  to  slates  or  to  shales.  Failures 
have  occurred  by  the  rock  itself  breaking  under  the  lateral  pressure 
exerted.  A  mass  of  concrete  of  sufficient  weight  to  resist  sliding, 
itself  keyed  and  fastened  to  the  rock  by  drift  bolts  for  increased 
safety,  is  recommended  for  the  foundation  of  all  dams.  De  Mas 
has  well  stated  that  in  river  construction  economies  should  be 
limited  to  the  work  above  water  which  can  be  observed. 

Next  to  rock,  a  gravel  bar  usually  is  considered  the  best  loca- 
tion for  a  dam,  and  it  is  unquestionably  true  that  gravel  offers  a 
greater  resistance  to  a  dangerous  subsurface  flow  than  sand  or 
silt.  However,  it  is  not  safe  to  make  assumptions  regarding  the 
difference,  and  foundations  based  on  such  assumptions  are  liable 
to  fail.  On  a  gravel  bar,  it  is  difficult  to  drive  the  wall  of  water- 
tight sheet-piling  which  it  is  usual  to  rely  upon  to  increase  the 
length  of  the  path  of  subsurface  flow  in  sand  or  silt,  and  dams  have 
been  constructed  on  a  timber  grillage  without  pile  support  in 
gravel.  On  the  Osage  River,  the  water  excavated  a  hole  under 
such  a  dam  several  years  after  its  completion,  which  carried  the 
entire  low- water  discharge;  and  a  section  of  a  dam  on  the  upper 
Mississippi  was  destroyed  before  the  cofferdam  was  removed. 
The  cause  of  such  failures  is  explained  in  Chapter  II. 

Clay  is  a  good  foundation  on  land  but  it  is  rarely  found  in 
alluvial  rivers  except  at  considerable  depths,  being  usually  so 
mixed  with  sand  as  to  produce  what  is  termed  silt.  When  the 
water-tight  wall  of  sheet  piling  can  be  driven  into  a  layer  of  im- 
permeable clay,  a  most  satisfactory  cut-off  wall  can  be  constructed 
between  the  two  pools,  and  the  danger  of  percolation  can  be 
eliminated. 

Sand  and  silt  are  recognized  as  unstable  foundations.  While 
numerous  failures  have  been  recorded  in  the  past,  a  stable  struc- 
ture can  be  erected  on  them  with  proper  precautions.  To  pre- 
vent settlement,  the  dam  is  supported  on  piles.  To  limit  perco- 
lation, a  water-tight  wall  of  sheet  piling  is  driven  at  the  upper 
face  of  the  dam.  It  is  usually  impracticable  to  obtain  a  clay 
sub-base  into  which  the  sheet  piling  can  be  driven  and  all  perco- 


74  RIVERS  AND   HARBORS 

lation  prevented,  but  the  velocity  of  the  flow  can  be  so  reduced  as 
to  be  incapable  of  moving  the  material  of  which  the  foundation  is 
composed. 

It  has  been  found  practically  in  levee  construction  on  the  Mis- 
sissippi River  that  if  the  water  is  forced  to  travel  through  the  al- 
luvium of  the  valley,  over  a  path  which  exceeds  ten  times  the  head, 
the  soil  acts  as  a  filter,  the  material  held  in  suspension  is  de- 
posited, and  the  outflowing  water  is  clear.  Occasionally,  how- 
ever, while  the  sediment  is  deposited,  sand  boils  will  appear  be- 
hind the  levee  line,  indicating  that  the  water  still  has  sufficient 
force  to  move  small  particles  of  sand.  If  a  sub-levee  is  built 
behind  the  main  line  so  as  to  reduce  the  head  to  1  in  15  as  the 
space  between  the  levee  lines  fills  with  water,  this  motion  of  sand 
ceases.  Experiments  of  a  similar  character  have  demonstrated 
that  if  the  head  can  be  reduced  to  one  foot  in  twenty,  a  scouring 
flow  through  the  fine  silt  of  the  rivers  of  India  can  be  prevented. 

In  levee  construction,  the  necessary  length  of  path  to  produce 
the  proper  relation  to  the  head  usually  is  obtained  by  extending 
the  levee  base.  In  concrete  dam  construction  this  method  is  too 
expensive,  and  the  water  usually  is  forced  to  a  path  of  the  proper 
length,  by  constructing  under  the  dam  a  water-tight  wall  of  sheet 
piling  whose  tops  are  imbedded  in  the  concrete  of  the  base.  When 
such  a  wall  is  used,  the  length  of  the  path  of  the  flowing  water  is 
computed  as  along  the  base,  vertically  down  one  side  of  the  sheet 
piling  and  up  the  other  side.  If  a  second  wall  is  constructed 
under  the  dam  it  should  be  spaced  at  least  twice  the  depth  of  the 
pile  penetration  from  the  first  wall.  This  form  of  structure  is 
not  usually  favored  by  American  engineers,  since  the  second  line 
of  sheet  piling  increases  the  water  pressure  under  the  dam. 

Another  danger  has  recently  been  discovered  in  the  investiga- 
tions of  the  Miami  Conservation  Commission  (5).  If  the  water 
contains  certain  amounts  of  animal  or  vegetable  matter,  gas  gen- 
erated by  decomposition  is  liable  to  accumulate  in  the  space  be- 
tween the  walls  and  to  cause  a  considerable  variation  in  the  com- 
puted pressures. 

Another  method  of  increasing  the  length  of  path  to  be  followed 
by  the  water  is  to  deposit  over  the  bed  above  the  dam  a  layer  of 
impervious  clay.  Nature  frequently  reduces  the  percolation  under 
dams  by  making  such  deposits,  and  in  the  experiments  on  the 
Miami  River  referred  to  above,  the  natural  deposits  of  silt  were 


RIVER   IMPROVEMENT   BY   CANALIZATION  75 

more  efficient  in  reducing  the  upward  pressure  under  the  dam 
than  the  walls  of  sheet  piling.  In  a  dam  constructed  on  the 
Ouachita  River,  it  was  attempted  to  combine  the  two  methods 
and  to  reduce  the  amount  of  penetration  required  of  the  sheet 
piling  cut-off  wall  by  a  covering  of  clay  above  the  dam.  The 
attempt  was  unsuccessful,  and  a  second  line  of  sheet  piling  had 
to  be  driven  to  prevent  a  dangerous  subsurface  flow. 

The  water  flowing  over  the  top  of  a  dam  is  also  a  source  of 
danger.  Even  when  dams  are  founded  on  rock,  the  impact  of 
the  falling  water  frequently  will  wear  away  the  rock  foundation. 
The  dam  constructed  across  the  Mississippi  River  at  Keokuk, 
Iowa  (6),  in  1913  has  already  required  a  concrete  protection  to 
portions  of  its  foundations.  Where  the  foundation  is  gravel  or 
sand,  special  care  must  be  taken  to  prevent  the  overflow  from 
undermining  the  dam.  To  receive  the  impact  of  the  water,  an 
apron  is  constructed  which  may  be  composed  of  heavy  rip-rap, 
or  which  may  be  a  timber  crib  filled  with  stone  and  decked  over 
with  timbers  securely  fastened  to  the  structure.  A  layer  of  con- 
crete frequently  is  substituted  for  the  timber  floor.  The  ratio  of 
the  width  of  the  apron  to  the  height  of  the  dam  is  usually  from 
one  and  one-half  to  two;  but  when  the  bed  of  the  river  is  a  fine 
silt,  as  is  the  case  at  certain  dams  in  India,  the  rip-rap  apron  may 
extend  for  long  distances.  The  Dauleshwiram  dam  (7),  with  a 
crest  head  of  16.8  feet,  has  an  apron  185  feet  long.  The  Laguna 
dam  built  by  the  Reclamation  service  in  Arizona,  with  a  crest 
19  feet  above  the  river  bed,  is  a  rock-filled  dam  with  three  concrete 
cut-off  walls.  The  apron  is  combined  with  the  dam  and  given  a 
surface  of  concrete  of  uniform  slope.  The  total  width  of  the  dam 
is  244  feet.  The  Mahanuddee  weir  in  India,  a  similar  structure 
whose  crest  elevation  is  13  feet,  and  whose  base  width  is  173  feet, 
was  seriously  damaged  during  a  flood. 

The  same  precautions  must  be  taken  to  prevent  percolation 
around  the  abutments  of  a  dam  as  under  its  foundation.  There 
is  also  usually  a  certain  contraction  of  the  waterway  at  the  site 
of  the  dam  which  induces  eddy  action  below  it  and  necessitates 
bank  protection. 

There  has  been  considerable  discussion  of  the  proper  location 
of  dams  provided  for  river  improvement,  whether  they  should  be 
placed  in  straight  reaches  of  a  river  or  in  bends,  and  if  in  bends 
whether  the  lock  should  be  on  the  concave  or  convex  bank.  A 


76  RIVERS  AND   HARBORS 

dam  should  have  as  long  a  crest  as  practicable  to  reduce  flood 
heights,  but  it  may  be  more  economically  constructed  in  straight 
narrow  river  sections.  The  entrance  to  a  lock  on  a  convex  bank 
is  more  readily  obstructed  by  sand,  and  one  on  the  concave  bank 
by  drift.  The  deciding  questions  on  lock  and  dam  construction, 
however,  are  usually  the  character  of  the  foundation  and  the 
topography  of  the  site.  The  ideal  location  for  a  lock  is  on  one 
side  of  an  island,  with  the  waste  weir  of  the  dam  on  the  other  side. 

With  a  small  low-water  discharge,  it  is  sometimes  necessary  to 
substitute  a  lateral  canal  for  the  natural  river  channel,  and  to 
utilize  the  river  merely  as  a  feeder.  This  is  the  only  practical 
method  of  constructing  a  navigable  channel  through  the  area  of 
deposition.  The  lateral  canal  also  affords  a  convenient  method 
of  surmounting  rapids.  When  a  river  has  an  ample  low- water 
discharge,  and  a  gentle  slope,  the  substitution  of  a  lateral  canal  for 
river  regulation  is  inadvisable  not  only  on  account  of  the  increased 
cost,  but  also  on  account  of  the  necessarily  contracted  section  of 
the  waterway.  The  resistance  of  a  boat  to  motion  is  much  less 
in  a  wide  river. 

Dams  convert  the  river  channel  into  a  series  of  lakes,  and  the 
flow  of  sediment  instead  of  conforming  to  that  in  an  unrestrained 
alluvial  stream  resembles  that  which  exists  in  valleys  formed  by 
glacial  action.  The  material  which  is  rolled  along  the  river  bed 
as  sand  waves  is  deposited  in  the  upper  pool  and  may  be  insufficient 
to  fill  it  for  a  long  period.  In  the  remaining  portions  of  the  canal- 
ized bed,  increased  depths  may  result,  since  the  normal  supply 
of  material  in  motion  has  been  reduced.  Material  in  suspension 
is  deposited  further  downstream.  Its  deposition  resembles  that 
which  occurs  on  the  banks  of  an  alluvial  river  during  floods,  and 
is  the  greatest  where  the  velocity  is  first  checked,  and  gradually 
diminishes  as  the  supply  of  suspended  material  is  exhausted. 
The  downstream  slope  of  a  deposit  from  a  sand  wave  is  relatively 
steep,  while  that  of  a  deposit  derived  from  material  in  suspension 
is  very  gentle,  not  infrequently  averaging  as  little  as  one  foot  per 
mile. 

There  is  a  shoaling  of  the  channel  and  an  increase  in  river 
slopes  wherever  the  deposit  occurs,  and  either  periodic  dredging 
is  necessary  at  such  localities  to  maintain  the  depths  required  for 
navigation,  or  the  channel  must  be  contracted  so  as  to  remove 
the  deposits  by  scour.  The  works  of  contraction  also  must  be 


RIVER   IMPROVEMENT   BY   CANALIZATION  77 

periodically  extended,  as  deposits  form  in  the  pool  below  them. 
Hence  if  regulation  is  relied  upon  to  maintain  the  channel,  the 
regulation  dikes  must  ultimately  extend  through  the  entire  section 
of  the  river  that  is  canalized,  even  when  the  changes  in  slope  caused 
by  the  construction  of  the  dams  have  removed  all  tendency  to  bank 
caving  in  the  river  bed. 

For  this  reason  a  river  flowing  through  glacial  drift  can  be  im- 
proved by  canalization  more  readily  than  one  flowing  in  an  alluvial 
valley.  In  Chapters  II  and  IV  attention  was  invited  to  the 
relatively  small  erosive  power  of  the  water  of  glacial  rivers,  and 
to  the  tendency  of  such  rivers  to  flow  with  gentle  slopes,  until 
obstructed  by  rock  ridges  or  by  moraine  deposits  over  which  the 
fall  is  concentrated  in  rapids  or  cataracts.  The  canalization  of 
these  short  obstructed  reaches  frequently  will  create  a  navigable 
channel  for  long  distances,  from  which  the  deposits  of  sand  waves 
can  be  dredged  economically.  Thus  in  the  St.  Lawrence  Valley, 
as  shown  in  Fig.  3,  the  greater  portion  of  the  slope  is  concentrated 
in  the  Niagara  River,  in  which  the  fall  is  about  315  feet,  and  in 
the  upper  St.  Lawrence  River,  in  which  the  fall  is  about  216  feet. 
The  Canadian  Government  has  developed  a  navigable  waterway 
of  fourteen  feet  depth  from  Montreal  to  Lake  Erie,  by  constructing 
a  system  of  lateral  canals  around  these  obstacles,  and  the  United 
States  Government  has  created  and  readily  maintains  a  navigable 
channel  of  twenty-one  feet  depth  from  the  foot  of  Lake  Erie  to 
Chicago,  a  distance  of  about  900  miles  by  rock  excavation  and 
dredging  in  the  waters  connecting  Lake  Huron  and  Lake  Erie, 
in  which  the  fall  is  only  nine  feet. 

In  the  waterway  from  Chicago  to  the  Gulf  of  Mexico,  the 
slopes  are  more  uniformly  distributed  than  in  the  St.  Lawrence 
Valley.  Its  canalization,  therefore,  would  necessitate  the  con- 
struction of  a  series  of  dams  between  Chicago  and  the  mouth  of 
the  Red  River.  The  pools  thus  formed  in  the  glacial  valleys  of 
either  the  Desplaines  or  the  Illinois  rivers,  would  not  tend  to 
fill  with  sediment,  as  they  should  derive  their  water  supply  from 
Lake  Michigan.  Below  the  mouth  of  the  Missouri,  however,  all 
attempts  to  maintain  navigation  by  dredging  would  be  futile  on 
account  of  the  amount  of  sediment  which  is  carried  by  that  river 
at  all  stages,  and  deposited  whenever  the  velocity  of  the  current 
is  diminished.  The  estimated  yearly  discharge  of  sediment  by 
the  Mississippi  River  is  400,000,000  cubic  yards.  Some  200,000,000 


lit*        i        t 


:  2  I  I  *ssss 


RIVER   IMPROVEMENT  BY   CANALIZATION  79 

cubic  yards  would  be  deposited  annually  even  if  the  dams  were 
limited  in  height  to  a  bank  full  stage.  It  therefore  would  be 
necessary  not  only  to  create  the  pools  by  canalization,  but  to 
contract  them  by  regulation  by  the  amount  which  is  necessary 
to  maintain  the  velocity  of  flow  that  exists  in  the  unimproved 
river.  Such  contraction  would  also  reduce  to  a  minimum  the 
storage  capacity  of  the  reservoirs  which  are  created  by  the 
dams,  and  limit  their  capacity  to  produce  a  continuous  water 
power. 

The  advocates  of  the  canalization  of  the  river  claim  that  the 
large  water  power  developed  would  justify  the  enormous  cost  of 
the  project,  and  appear  to  base  their  estimates  on  maximum  or 
mean  river  discharges.  Such  discharges  could  not  be  utilized  for 
power  production  on  the  lower  Mississippi  River.  A  marked  in- 
crease in  flood  heights  from  dam  construction  could  not  be  per- 
mitted, since  the  crests  of  the  levees  along  certain  portions  of  the 
river  have  an  average  elevation  of  about  20  feet  above  the  surface 
of  the  ground  under  existing  conditions.  It  would,  therefore,  be 
necessary  to  produce  the  change  in  river  slopes  by  numerous  low 
dams.  Such  dams  at  high  stages  would  be  drowned  out. 

In  the  development  of  water  power  a  certain  velocity  must  be 
produced  in  the  water  wheel  to  create  a  given  electrical  energy, 
and  the  wheels  therefore  are  designed  for  an  assumed  head.  A 
uniform  velocity  can  be  maintained  with  small  variations  in  head 
by  regulating  the  flow  through  the  intake,  but  such  adjustments 
are  impossible  when  the  heads  vary  between  30  feet  and  zero. 
The  drowning  out  of  the  dam  will  suspend  the  production  of  elec- 
tric power.  A  power  derived  intermittingly  from  water  is  of 
little  value,  since  a  steam  plant  must  be  installed  for  use  during 
the  period  that  it  is  not  in  operation  and  this  plant  could  also 
operate  when  the  water  power  was  available.  The  saving  in  the 
coal  bill  while  the  steam  plant  is  idle  is  offset  by  the  interest  on 
the  cost  of  the  water  power  plant.  In  the  St.  Lawrence  Valley 
variations  between  high  and  low  water  are  small  and  can  be  regu- 
lated readily,  since  the  water  wheels  which  are  installed  operate 
at  high  heads. 

When  the  principles  governing  the  regulation  of  rivers  were  not 
properly  understood,  and  dredging  was  relatively  expensive,  a 
school  of  engineers  arose  which  advocated  canalization  as  the  only 
method  of  improving  non-tidal  rivers,  but  it  is  recognized  at  the 


80  RIVERS  AND  HARBORS 

present  time  that  "the  navigability  of  rivers  having  but  one  cur- 
rent can  be  improved  as  it  has  been  stated  many  times  at  the 
Navigation  Congresses  by  various  methods  such  as:  Regulation 
of  the  bed  by  permanent  works;  regulation  of  the  bed  by  me- 
chanical dredging;  increase  of  depth  by  an  additional  water  supply 
furnished  by  storage  reservoirs;  canalization  of  the  bed;  combined 
action  of  two  or  more  of  these  processes;  construction  of  a  lateral 
canal.  The  use  of  one  of  these  methods  rather  than  another  de- 
pends upon  the  special  circumstances  of  each  particular  case." 
(PERMANENT  ASSOCIATION  OF  NAVIGATION  CONGRESSES,  REPORT 
OF  PROCEEDINGS  OF  THE  XIlTH  CONGRESS  1912,  p.  386.) 

An  excessive  deposit  of  material  in  suspension  during  the  period 
in  which  it  is  necessary  to  maintain  the  dams  for  navigation,  a 
large  discharge  which  creates  a  river  of  great  width,  the  difficulty 
of  securing  suitable  foundations  for  the  dams,  low  banks  through 
which  there  is  a  seepage  injurious  to  crops  when  the  dams  are 
raised,  are  important  factors  in  determining  whether  a  river  should 
be  improved  by  regulation  or  by  canalization.  The  Rhine,  the 
Danube,  and  the  Po  rivers  in  Europe  and  the  Mississippi  River 
in  the  United  States  can  be  improved  by  regulation  and  dredging 
to  the  desired  depth  more  economically  than  by  canalization. 

On  the  other  hand,  the  slope  of  a  river  may  be  so  great  that, 
notwithstanding  the  fact  that  its  discharge  may  be  sufficient  to 
insure  adequate  depths,  the  necessary  contraction  by  regulating 
dikes  may  increase  the  velocity  of  its  currents  to  such  an  extent 
that  the  force  required  for  towing  boats  upstream  becomes  greater 
than  that  required  for  hauling  a  railway  train  of  the  same  carry- 
ing capacity. 

The  Rhone  has  been  improved  by  regulation  to  a  low-water 
depth  of  two  meters.  The  slopes  on  this  river  are  excessive, 
however,  and  make  up-bound  navigation  difficult.  Notwith- 
standing the  success  of  the  improvement,  canalization  has  recently 
been  proposed  with  dams  of  such  lifts  that  water  power  can  be 
developed  from  them.  It  is  claimed  that  the  power  development 
would  pay  for  the  cost  of  construction.  In  the  revised  project  for 
improving  the  upper  Mississippi  River,  two  short  lateral  canals  with 
locks  are  being  constructed  around  the  Rock  Island  rapids  for  the 
benefit  of  up-bound  navigation,  although  the  channel  through  the 
rapids  is  retained  for  down-bound  traffic. 

It  can  be  stated  as  a  general  law  that  mechanical  dredging 


RIVER   IMPROVEMENT   BY   CANALIZATION  81 

usually  affords  the  most  economical  method  of  maintaining  a 
channel  of  moderate  depth  near  the  mouths  of  large  rivers  (see 
Chap,  xii),  that  when  compared  with  dredging  the  importance  of 
regulation  as  a  means  of  improvement  increases  as  the  low-water 
discharge  diminishes  and  the  slope  increases  (see  Chap,  ix),  that 
a  further  increase  in  slope  may  render  canalization  more  economical, 
and  that  the  substitution  of  a  lateral  canal  for  a  canalized  river 
is  justifiable  under  certain  conditions,  notwithstanding  the  in- 
creased cost  of  towing  within  its  contracted  waterway.  Where 
rapids  make  navigation  difficult  a  lateral  canal  may  be  con- 
structed whose  initial  cost  may  be  less  than  that  of  a  dam  extend- 
ing across  the  river.  If.  however,  water  power  can  be  created 
by  building  the  dam,  its  utilization  may  make  the  canalization 
of  the  river  bed  a  more  profitable  investment.  The  Keokuk 
rapids  in  the  Mississippi  River  were  surmounted,  formerly,  by  a 
lateral  canal.  A  high  masonry  dam  (6),  however,  has  been  con- 
structed across  the  river  recently,  which  not  only  develops  water 
power,  but  also  facilitates  the  movement  of  vessels  on  account  of 
the  substitution  of  a  single  lock  with  a  high  lift  for  the  locks  of 
moderate  lift  in  the  lateral  canal.  When  the  banks  are  so  low 
that  movable  dams  would  cause  the  overflow  of  the  land  along  the 
river  or  cause  a  seepage  that  would  be  injurious  to  agriculture, 
the  lateral  canal  may  be  necessary.  The  discharge  of  the  river 
determines  whether  regulation  of  the  river  bed  or  the  construction 
of  a  lateral  canal  is  preferable. 

The  fact  that  a  dam  built  across  the  bed  of  an  alluvial  stream 
will  cause  a  deposit  of  the  detritus  which  is  transported  down  its 
valley,  is  of  importance  not  only  in  river  canalization  but  also  in 
reservoirs  constructed  for  power  development  and  for  furnishing 
cities  with  a  water  supply.  This  deposition  continues  until  the 
reservoir  capacity  is  reduced  to  such  an  extent  that  the  stream 
will  flow  with  sufficient  velocity  through  the  reservoir  to  transport 
its  sediment,  and  ordinarily  if  the  reservoir  capacity  is  to  be 
maintained,  the  cheapest  method  of  removing  the  deposits  is  by 
dredging. 

If  the  detritus  moves  in  sand  waves  and  in  relatively  small 
amounts,  an  auxiliary  dam  can  be  constructed  at  the  upper  end 
of  the  reservoir  which  will  retain  the  sediment  above  it  for  a 
limited  period;  and  by  connecting  the  upper  pool  thus  formed  by 
large  conduits  to  the  portion  of  the  stream  below  the  main  dam, 


82  RIVERS  AND   HARBORS 

the  deposits  can  be  transported  around  the  reservoir  through  the 
conduits  by  repeated  flushings.  The  deposition  extends  over  the 
entire  reservoir  bed  when  a  large  percentage  of  the  material  is 
carried  in  suspension,  and  to  attain  the  same  object  the  conduits 
must  be  located  in  the  vicinity  of  the  main  dam.  In  a  canalized 
river,  the  entire  pool  is  periodically  flushed  by  manipulating 
movable  dams  which  can  be  quickly  lowered;  but  such  a  procedure 
destroys  the  reservoir  capacity,  and  is  therefore  inapplicable 
when  the  reservoir  is  required  to  store  water.  No  system  of 
sluice  gates  would  be  effective  for  scour  which  did  not  create  a 
greater  velocity  through  the  reservoir  than  existed  in  the  river 
channel  before  the  dam  was  erected,  because  it  requires  a  greater 
velocity  to  put  in  motion  material  deposited,  than  it  does  to 
transport  it  when  in  suspension. 

Theoretically  a  series  of  conduits  could  be  constructed  in  the 
bed  of  a  reservoir  with  numerous  openings,  and  by  allowing  these 
conduits  successively  to  discharge  below  the  dams  during  high 
stages,  a  local  scour  could  be  created  in  the  vicinity  of  each  which 
would  remove  the  material  surrounding  it  without  materially 
lowering  the  water  level  of  the  reservoir.  The  scour  is  limited, 
however,  to  a  short  distance  from  the  opening  unless  it  is  very 
large.  Practically,  in  such  a  system  of  conduits,  the  openings 
would  be  so  large  and  numerous  that  the  conduits  would  become 
choked  with  sediment  during  deposition  in  the  reservoir  and  fail 
to  work  when  needed,  just  as  a  sewer  pipe  clogs  when  it  receives 
an  intermittent  flow  without  a  sufficiently  constant  discharge  to 
prevent  the  deposition  of  sewerage. 

It  is  practicable  to  largely  reduce  the  deposition,  where  only 
a  limited  portion  of  the  river  discharge  is  utilized  and  the  reservoir 
can  be  constructed  in  a  portion  of  the  valley  outside  the  stream 
bed.  By  building  a  weir  of  sufficient  height  in  its  intake  the 
sand  waves  rolling  along  the  river  bed  can  be  entirely  prevented 
from  entering  the  reservoir.  By  means  of  sluice  gates  on  the  weir 
which  are  opened  only  when  the  river  carries  little  material  in 
suspension,  this  cause  of  deposition  can  be  greatly  reduced.  By 
such  means,  however,  only  a  small  percentage  of  the  river  dis- 
charge can  be  utilized.  If  the  water  is  to  be  taken  from  the  river 
at  all  stages,  a  settling  basin  must  be  constructed  above  the  weir, 
which  will  cause  a  partial  settlement  of  material  in  suspension 
before  it  approaches  the  intake. 


RIVER   IMPROVEMENT   BY   CANALIZATION  83 

While  frequently  the  degree  of  saturation  of  water  carrying 
material  in  suspension  diminishes  from  the  bed  of  a  river  to  its 
surface,  this  is  not  an  invariable  rule.  In  discussing  the  critical 
stage  in  Chapter  III,  attention  was  invited  to  the  effect  of  sand 
waves  on  the  flow  of  water  during  floods,  and  the  boils  and  eddies 
which  they  produced  at  the  water  surface.  Velocities  then  are 
generated  capable  of  carrying  gravel  in  suspension,  as  is  shown  by 
the  deposits  of  gravel  at  high  stages  along  the  banks  of  alluvial 
streams.  When  such  boils  appear,  the  upper  portions  of  the 
river  are  carrying  a  greater  load  of  sediment  than  the  waters  at 
greater  depths.  This  heavy  material,  however,  is  deposited 
readily  and  a  settling  basin  of  limited  extent  will  cause  a  clarifi- 
cation of  the  surface  waters.  Even  though  a  sufficient  time  has 
not  elapsed  to  cause  the  material  to  be  deposited  on  the  river  bed, 
the  sediment  deposited  in  the  reservoir  will  be  reduced  to  a  mini- 
mum, if  a  weir  of  great  length  is  constructed  and  its  gates  are 
so  manipulated  that  only  a  thin  sheet  of  surface  water  flows  over  it. 
This  principle  is  utilized  in  clarifying  the  water  supply  of  cities, 
and  reducing  the  amount  of  deposits  requiring  removal  from 
filtration  basins. 

On  the  Great  Lakes,  the  bacteriological  condition  of  the  water 
supply  of  cities  is  greatly  improved  by  the  proper  manipulation 
of  valves  in  their  intake  towers  (8).  While  the  flow  of  sediment  in 
the  deep  water  at  the  intakes  is  insignificant,  a  considerable 
amount  of  sewerage  empties  into  the  lakes  and  pollutes  their 
waters.  With  a  wind  blowing  from  the  shore,  this  sewerage 
flows  on  the  water  surface  and  the  pure  lake  waters  flow  along 
the  lake  bed.  When  the  direction  of  the  wind  is  reversed,  the 
sewerage  flows  along  the  bed  and  pure  water  on  the  surface.  The 
intake  gates  to  the  water  supply  tunnels  can  be  manipulated 
accordingly. 

The  most  extensive  application  of  these  principles  has  been 
made,  however,  in  the  irrigation  ditches  which  derive  their  waters 
from  the  rivers  of  India,  carrying  an  abnormal  amount  of  silt. 
The  Sirhind  Canal  (9),  which  derives  its  waters  from  the  Sutlej 
River  at  Rupar,  is  a  conspicuous  example.  The  Sirhind  Canal 
has  a  width  of  200  feet  and  a  depth  of  10  feet  and  was  designed 
for  a  discharge  of  7,000  second-feet.  To  maintain  the  water  of 
the  Sutlej  River  at  a  proper  height,  a  dam  was  constructed  across 
the  river  below  the  inlet  to  the  canal.  The  unregulated  waters 


84  RIVERS  AND   HARBORS 

from  the  river  soon  filled  the  upper  portions  of  the  canal  with 
sediment  to  such  an  extent  as  to  seriously  reduce  its  discharge. 
It  was  first  attempted  to  reduce  the  deposit  by  increasing  the 
velocity  in  the  canal  by  permitting  a  discharge  through  sluices 
situated  about  twelve  miles  from  the  intake.  It  is  stated  (9) 
that  this  did  some  good,  but  that  there  seldom  was  water  to  spare 
for  the  purpose.  A  regulating  sill  was  then  raised  to  7  feet  above 
the  canal  bed  on  which  shutters  were  constructed  which  could 
further  increase  the  elevation  three  feet.  Material  in  suspension 
passed  over  the  sill,  and  it  was  necessary  to  keep  the  canal  closed 
during  heavy  floods.  A  divide  wall  710  feet  long  was  then  con- 
structed perpendicularly  to  the  river  dam  so  as  to  create  a  pool 
between  it  and  the  regulator.  A  rapid  deposit  of  sediment  oc- 
curred in  this  pool  which  could,  however,  be  flushed  out  fre- 
quently through  sluices  in  the  dam.  With  a  proper  flushing  of  this 
pool,  the  period  in  which  water  could  be  introduced  into  the 
canal  was  increased,  but  the  supply  to  the  canal  was  usually  sus- 
pended while  the  pool  was  being  flushed. 

Three  methods  of  raising  or  lowering  a  boat  to  enable  it  to 
overcome  the  difference  in  elevation  above  and  below  a  dam  have 
been  devised  (10).  The  incline,  the  lift,  and  the  lock.  With  the 
incline  the  boat  enters  a  water-tight  caisson  which  is  supported 
on  a  carriage  mounted  on  wheels.  This  carriage  moves  on  rails 
which  frequently  are  built  with  a  slope  of  1  to  10.  Usually  two 
caissons  and  carriages  are  constructed,  which  with  their  loads 
counterbalance  one  another.  They  are  connected  by  a  cable, 
and  one  ascends  as  the  other  descends,  the  power  necessary  to 
overcome  frictional  resistance  is  supplied  by  an  engine  driven  by 
steam,  water  power,  or  electricity. 

With  the  lift,  the  caisson  is  raised  vertically,  usually  by  means 
of  hydraulic  rams,  though  in  some  of  the  earlier  designs  caissons 
counterbalancing  one  another  and  connected  by  cables  were  em- 
ployed as  in  inclines. 

Except  where  large  differences  of  elevation  are  to  be  overcome, 
the  lock  is  employed  universally  to  pass  from  one  elevation  to  the 
other.  A  lock  consists  of  a  chamber  in  which  the  elevation  of 
the  water  surface  can  be  changed  from  that  of  the  upper  pool  to 
that  of  the  lower  one,  and  vice  versa.  There  are  upper  and  lower 
gates,  which  when  opened  permit  the  boat  to  enter  and  depart  from 
the  chamber.  When  the  gates  are  closed,  sluices  regulated  by 


RIVER   IMPROVEMENT   BY   CANALIZATION  85 

valves  allow  a  flow  of  water  into  or  out  of  the  chamber,  so  that  the 
water  surface  can  be  raised  or  lowered  as  desired.  The  invert  of  the 
lock-chamber  and  the  sills  of  the  lower  gates  must  have  such  a 
depth  of  water  over  them  that  vessels  can  pass  from  the  lower 
pool  into  and  out  of  the  lock,  and  the  elevation  of  the  top  of  the 
chamber  walls  must  exceed  that  of  the  upper  pool.  In  canal 
locks,  the  upper  gates  are  supported  by  a  lift-wall  which  extends 
from  the  elevation  of  the  bed  of  the  lower  pool  to  that  of  the 
upper  one.  In  locks  closing  tidal  basins,  the  upper  and  lower 
gates  are  usually  of  the  same  dimensions. 

As  much  ingenuity  has  been  expended  in  building  lock-gates  as 
in  constructing  movable  dams.  In  fact,  many  of  the  types  of 
movable  dams  have  been  employed  also  as  lock-gates.  The 
tumble-gates  of  the  locks  of  the  Old  Erie  Canal  work  on  the  same 
principle  as  shutters,  which  rotate  around  a  horizontal  axis  and 
rest  on  the  lock  floor  when  open.  The  gates  of  the  Elbe-Trave 
Canal  in  Germany  have  hollow  chambers.  When  air  is  forced 
into  the  chambers  in  a  gate  it  rotates  about  its  axis  in  a  manner 
similar  to  the  modified  Brunot  shutter.  The  latter  is  moved  by 
hydraulic  pressure  under  the  caisson,  however,  instead  of  by 
compressed  air.  The  Chittenden  drum  weir  has  been  reproduced 
as  a  gate  in  a  lock  on  the  Mississippi  River  at  St.  Paul.  Bear- 
trap  gates  have  been  constructed  at  several  localities.  A  roller- 
lift  gate  has  been  constructed  for  the  lock  on  the  New  York  Barge 
Canal  at  Little  Falls,  which  is  raised  to  such  a  height  that  boats 
can  pass  under  it.  Roller  gates,  which  slide  horizontally  into 
recesses  in  the  lock  walls,  have  also  been  built,  particularly  in 
locks  on  the  Ohio  River. 

The  gates  most  often  employed  are  those  which  rotate  about  a 
vertical  axis.  For  narrow  locks,  a  single  leaf  is  employed  which 
rotates  around  a  quoin  post  located  in  a  recess  of  the  lock-wall 
made  to  fit  it,  and  called  the  hollow  quoin.  The  leaf  is  balanced 
by  a  timber  arm  on  the  land  side  of  the  quoin.  This  arm  is  used 
also  to  open  and  to  close  the  leaf.  When  closed  the  gate  abuts 
against  a  sill  along  its  lower  edge  and  against  shoulders  of  the 
lock-wall.  This  system  is  used  extensively  in  the  canal  locks  of 
France.  For  larger  locks  in  the  United  States,  two  gates  are 
employed,  each  of  which  has  a  greater  width  than  one-half  the 
lock,  so  that  the  gates  when  closed  incline  upstream  and  abut 
against  one  another  along  what  are  termed  mitering  posts.  Since 


86  RIVERS  AND   HARBORS 

the  strains  to  which  the  gates  are  subjected  vary  with  the  incli- 
nation between  them  and  with  their  form,  whether  straight  or 
curved,  the  inventive  genius  of  the  country  has  produced  numerous 
varieties  of  mitered  gates  whose  advantages  have  been  elaborately 
discussed  (11). 

Lock-gates  are  subject  not  only  to  ordinary  wear  and  tear,  but 
also  to  injury  from  a  vessel's  striking  them  as  it  enters  the  lock, 
and  some  device  is  necessary  to  retain  the  water  of  the  upper  pool 
at  its  proper  level  while  an  upper  gate  is  being  repaired.  For 
repairs  to  the  lock-chamber,  a  similar  device  is  required  to  exclude 
the  waters  of  the  lower  pool.  For  narrow  locks  stop-planks 
placed  in  grooves  in  an  extension  of  the  lock-walls  usually  are 
employed  for  this  purpose.  The  floating  caisson  employed  in 
dry  docks  is  better  adapted  to  wide  locks.  In  the  first  American 
canal  constructed  at  Sault  Ste.  Marie,  two  gates  abutting  against 
a  mid-channel  pier  were  first  employed  to  maintain  the  upper 
level,  while  the  lock  was  being  repaired,  but  the  frequency  of 
collisions  by  boats  with  the  lock  gates  led  the  Canadian  Govern- 
ment, when  it  constructed  its  lock  and  canal,  to  substitute  for 
the  gates  an  emergency  dam  operated  from  a  swing-bridge,  so 
that  even  if  a  gate  is  destroyed  by  a  collision,  the  flow  of  water 
can  be  stopped.  The  United  States  Government  has  constructed 
a  similar  structure  on  the  pier  between  the  two  protective  gates. 
The  wisdom  of  this  policy  was  demonstrated  when  one  of  the 
gates  of  the  Canadian  lock  was  carried  away  by  a  collision,  and 
the  flow  through  the  lock  had  to  be  checked.  In  the  third  and  fourth 
locks  of  the  American  canal  at  Sault  Ste.  Marie  (12),  two  sets 
of  upper  and  lower  gates  have  been  substituted  for  the  emergency 
dam.  These  are  made  so  strong  that  the  momentum  of  the  boat 
will  be  checked  by  the  destruction  of  one  gate,  and  both  the 
upper  or  the  lower  sets  are  always  mitered  before  a  boat  is  per- 
mitted to  approach  the  lock.  An  emergency  swing-bridge  dam 
has  been  constructed  to  protect  the  locks  of  the  Panama  Canal 
(13).  The  gates  are  further  protected  by  heavy  chains  which  are 
swung  across  the  channel  to  receive  the  impact  of  a  boat  not 
under  proper  control  and  to  reduce  its  momentum. 

Locks  are  filled  and  emptied  either  through  valves  in  the  gates, 
or  through  conduits  located  in  the  walls  or  under  the  lock  floor 
and  controlled  by  valves.  The  valves  are  of  numerous  types. 
Those  most  often  used  are  (a)  balanced  or  butterfly  valves 


RIVER   IMPROVEMENT   BY   CANALIZATION  87 

rotating  about  a  horizontal  axis  or  about  a  vertical  axis,  (b) 
sliding-valves  which  when  large  are  moved  on  rollers  as  are 
Stoney  gates,  and  (c)  drum-valves.  In  small  locks  that  are  filled 
slowly  the  location  of  the  filling  and  emptying  conduits  is  of  minor 
importance;  but  in  large  locks  which  require  the  sudden  addition 
or  removal  of  large  amounts  of  water,  conduits  under  the  lock- 
floor  with  numerous  openings  into  the  locks  produce  the  least 
surging  of  the  vessels  locking  through  them.  The  discharge  of 
a  large  lock  may  be  sufficient  to  require  a  concrete  apron  to  prevent 
scour  for  a  considerable  distance  below  the  lock.  The  discharge 
capacity  of  the  third  and  fourth  locks  (14)  at  the  Sault  Ste.  Marie  at 
maximum  head  and  with  the  gates  fully  opened,  exceeds  5000 
second-feet,  or  over  twice  the  low-water  discharge  of  the  Mississippi 
River  at  St.  Paul.  The  Poe  lock  at  Sault  Ste.  Marie  can  be  emp- 
tied or  filled  in  nine  minutes,  giving  an  average  discharge  during 
the  entire  operation  of  about  3000  second-feet.  When  both  the 
Weitzel  and  Poe  locks  are  being  filled  simultaneously,  a  current 
averaging  over  one  foot  per  second  is  created  in  the  upper  pool  of 
the  American  Canal,  and  a  series  of  oscillating  waves  are  produced 
which  have  received  careful  study  and  which  necessitate  special 
care  in  mooring  boats  waiting  their  turn  to  pass  through  the 
locks. 

The  width  to  be  given  a  lock  depends  on  the  character  of  the 
boats  which  are  to  pass  through  it.  When  single  vessels  are 
employed,  the  length  and  width  of  the  lock  should  readily  ac- 
commodate the  largest  vessel,  and  where  a  ship-building  industry 
is  situated  above  a  lock,  it  may  be  necessary  to  provide  for  vessels 
larger  than  those  which  ordinarily  navigate  the  river.  When 
tows  of  barges  are  employed,  it  is  desirable  to  pass  an  entire  tow 
at  a  single  lockage.  On  the  Ohio  River  the  locks  are  given  a  width 
of  100  feet  and  a  length  of  600  feet  in  order  to  pass  tow-fleets. 

It  is  also  necessary  to  take  into  consideration  the  future  growth 
of  commerce,  as  is  illustrated  by  lock-construction  at  Sault  Ste. 
Marie  (15).  The  Weitzel  lock  completed  in  1881  was  designed 
for  vessels  of  a  draft  of  16  feet.  It  was  60  feet  wide  at  the  gates 
and  80  feet  wide  in  the  chamber,  515  feet  long,  and  17  feet  deep. 
The  Canadian  lock,  completed  in  1895,  was  also  made  60  feet 
wide,  but  it  was  made  900  feet  long  to  accommodate  a  vessel 
and  its  tow,  with  the  depth  on  the  miter  sills  of  22  feet.  The 
Poe  lock,  which  was  designed  prior  to  1887  and  completed  in 


88  RIVERS  AND  HARBORS 

1896,  was  intended  to  accommodate  four  vessels  at  a  time.  It 
was  made  100  feet  wide,  800  feet  long,  and  22  feet  deep  on  the 
miter  sills.  When  it  was  designed,  the  largest  vessels  navigating 
the  lakes  had  a  length  of  350  feet  and  a  beam  of  45  feet.  The 
increase  in  depth  of  the  Poe  and  the  Canadian  locks  over  the 
Weitzel  lock  led  to  a  rapid  enlargement  of  the  dimensions  of 
vessels.  In  1903  there  were  97  vessels  navigating  the  lakes 
having  a  length  exceeding  400  feet  and  a  beam  from  45  to  53  feet. 
In  1918  there  were  42  vessels  of  a  length  exceeding  600  feet  and 
a  beam  from  58  to  64  feet,  and  both  the  Poe  and  Canadian  locks 
became  of  uneconomical  dimensions,  even  for  the  average  vessel 
that  navigated  the  lakes.  The  third  and  fourth  American  locks 
were  made  80  feet  wide,  1350  feet  long,  and  24|  feet  deep  on  the 
miter  sills,  so  as  to  accommodate  two  of  the  largest  vessels,  or 
three  of  average  size,  at  one  lockage. 


CHAPTER  IX 

DREDGING— REMOVAL  OF  OBSTRUCTIONS— BUOYS  AND  LIGHTS 

DREDGING 

It  frequently  happens  in  valleys  formed  wholly  or  in  part  of 
glacial  drift,  that  bars  exist  in  rivers  that  have  such  a  resistance 
to  erosion  that  they  cannot  be  removed  by  river  currents  even 
when  the  channel  is  contracted.  If  they  are  composed  of  gravel, 
clay  or  boulders,  a  channel  can  be  excavated  through  such  ob- 
structions by  dredging.  If  they  are  composed  of  rock,  it  is  usually 
necessary  to  resort  to  blasting.  Such  channels  are  usually  per- 
manent, since  the  rivers  flowing  in  such  valleys  transport  little 
sediment  as  sand  waves,  and  the  cuts  are  made  through  a 
material  incapable  of  being  scoured  by  the  gentle  currents  that 
exist. 

The  rivers  which  empty  into  the  Great  Lakes  (1)  and  their 
connecting  waters,  have  been  very  generally  improved  in  this 
manner,  but  it  must  be  borne  always  in  mind  that  the  tendency 
of  such  excavations  is  to  lower  the  level  of  the  pool  above  the  cut. 
The  mouths  of  many  rivers  emptying  into  the  Great  Lakes  have 
been  dredged  to  depths  of  from  12  to  23  feet,  and  the  natural 
low-water  river  slopes  have  been  completely  destroyed  thereby,  the 
depth  in  the  river  becoming  dependent  on  the  rise  and  fall  of  the 
lake  level. 

The  excavation  of  the  Neebish  Channels  in  the  St.  Mary's  River 
produced  a  lowering  of  the  water  surface  below  the  locks  at  Sault 
Ste.  Marie,  so  as  to  materially  diminish  the  draft  of  vessels  which 
could  pass  through  the  locks,  on  account  of  the  reduced  depth 
over  their  lower  miter  sills.  In  constructing  the  Livingston 
Channel  in  the  Detroit  River  (2),  this  lowering  of  the  upper  pool 
was  prevented  by  depositing  the  excavated  material  across  other 
portions  of  the  river  channel  so  as  to  limit  the  flow  through  shoal 
sections  of  the  river  sufficiently  to  compensate  for  the  increased 
flow  through  the  navigable  cut.  The  channels  excavated  in  the 
Rock  Island  rapids  (3)  of  the  Mississippi  River  exhibited  the  same 
tendency  toward  a  lowering  of  the  water  surface  of  the  upper 

89 


90  RIVERS  AND   HARBORS 

pools,  and  it  was  necessary  to  construct  not  only  a  series  of  long 
spur-dikes  to  maintain  the  pool  levels,  but  also  a  series  of  sub- 
merged sills  across  the  navigable  channel. 

When  a  river  is  canalized,  dredging  becomes  necessary  to  main- 
tain the  channel  through  the  deposits  of  material  carried  in  sus- 
pension or  rolled  along  the  river  bed,  which  form  below  the  dams. 
Under  the  conditions  described  above,  the  advantages  of  dredging 
are  self-evident. 

It  has  been  proposed  also  to  substitute  dredging  for  canaliza- 
tion, or  for  regulation  in  alluvial  rivers  carrying  large  amounts  of 
sediment.  The  limitations  of  dredging  under  such  circumstances 
require  explanation.  A  dredged  channel  is  subject  to  the  same 
laws  governing  the  flow  of  sediment  as  is  one  produced  by  natural 
scour.  In  the  straight  reaches  below  every  bend  there  is  a  move- 
ment of  sand  waves  and  an  elevation  of  their  crests  on  a  rising 
river.  Hence  there  is  a  tendency  for  a  dredged  cut  in  an  alluvial 
river  to  fill  on  every  rise,  requiring  re-dredging  as  the  river  falls. 
In  most  of  the  tributaries  whose  combined  flow  creates  the  main 
stream,  these  fluctuations  of  stage  are  frequent,  and  the  cut  must 
be  re-dredged  so  often  during  a  season  of  navigation  that  it  be- 
comes more  economical  to  direct  the  river  currents  by  means  of 
dikes  in  such  a  way  that  they  will  perform  the  same  work. 

The  fluctuations  of  the  tributaries  may  combine  in  the  main 
stream,  however,  in  such  a  way  as  to  produce  a  long  gradual  rise 
succeeded  by  a  slow  fall,  and  one  or  two  dredgings  of  the  bars  of 
the  main  river  may  be  sufficient  to  maintain  a  channel  during 
an  entire  year.  In  the  main  stream  the  cost  of  dike  construction 
largely  exceeds  that  in  the  tributaries  and  in  such  a  case  the  interest 
on  the  investment  in  dike  construction  may  exceed  the  cost  of 
maintenance  by  dredging.  The  tributaries  also  have  a  much 
smaller  low-water  discharge  than  the  main  stream  and  their 
slopes  are  usually  steeper. 

The  dimensions  of  the  channel  to  be  dredged  are  often  con- 
sidered a  function  of  the  size  of  the  vessels  which  it  is  proposed  to 
employ  for  navigation,  rather  than  of  the  discharge  and  the  slope 
of  the  river.  This  frequently  requires  in  a  tributary  a  channel 
of  such  width  and  depth  that  there  is  not  a  sufficient  discharge 
to  fill  it  during  low  water;  and  if  the  width  of  channel  is  reduced 
to  overcome  this  difficulty,  the  space  occupied  by  a  vessel  as- 
cending the  channel  becomes  so  great  as  to  interfere  seriously 


OBSTRUCTIONS  91 

with  the  river  flow,  and  the  boat  has  not  sufficient  power  to  propel 
itself  against  the  current  without  being  cordelled.1 

As  an  illustration,  let  us  assume  that  the  main  stream  has  a 
low-water  discharge  of  70,000  second-feet,  and  a  slope  over  its 
bars  of  0.4  foot  per  mile,  while  a  tributary  has  a  low-water  dis- 
charge of  1000  second-feet  and  a  local  slope  over  its  bars  of  1.5 
feet  per  mile.  In  the  main  river  a  channel  200  feet  wide  and  9  feet 
deep  will  have  a  cross-section  whose  area  is  1800  square  feet, 
and  will  discharge  about  5400  second-feet,  which  is  less  than 
eight  per  cent  of  the  flow  of  the  river.  In  the  tributary  a  navigable 
channel  6  feet  deep  will  require  a  width  of  at  least  70  feet  to  enable 
boats  to  pass  one  another.  The  area  of  its  cross-section  will 
therefore  be  at  least  420  square  feet,  and  its  discharge  will  exceed 
1400  second-feet  so  long  as  the  pool  above  can  supply  the  water, 
but  when  the  water  supply  has  become  exhausted  the  depth  of 
the  water  in  the  channel  diminishes.  Because  a  satisfactory  navi- 
gable channel  can  be  economically  dredged  on  the  lower  Missis- 
sippi River,  it  by  no  means  follows  that  a  similar  channel  can  be 
maintained  in  the  creeks  that  empty  into  it,  as  many  advocate. 

When  it  is  attempted  to  straighten  a  river  by  regulation,  the 
dredge  becomes  a  necessary  adjunct  to  the  improvement  during 
the  long  period  during  which  the  river  is  readjusting  its  slopes, 
since  channels  must  be  dredged  annually  through  the  sand-bars 
which  form  until  the  new  equilibrium  is  established. 

The  principles  governing  dredge  construction  (4)  belong  to 
mechanical  engineering,  and  attention  will  be  invited  here  only 
to  the  adaptability  of  certain  varieties  of  dredges  to  various 
kinds  of  work. 

The  dipper  dredge,  which  is  the  ordinary  steam-shovel  mounted 
on  a  barge,  is  the  best  machine  for  miscellaneous  dredging.  It 
can  excavate  either  hard  or  soft  material;  and  when  its  dipper  is 
armed  with  steel  teeth,  it  affords  the  most  economical  means  of 
removing  subaqueous  boulders  and  blasted  rock.  For  deep 
channels,  the  clam  shell  dredge  is  more  economical  when  the 
material  is  sufficiently  soft  to  be  excavated  by  its  bucket  and  is 
of  sufficient  consistency  to  remain  in  it  when  raised  to  the  surface. 

The  elevator  dredge,  though  rarely  employed  in  the  United 
States,  is  used  extensively  in  Europe  for  deep  channels.  When 
the  chain  of  buckets  is  made  particularly  strong,  it  can  excavate 

1 A  method  of  propulsion  by  a  vessel's  capstans  through  ropes  attached  to  trees  on  the  bank. 


92  RIVERS   AND   HARBORS 

stratified  rock.     It  was  employed  for  this  purpose  in  the  St. 
Lawrence  River  near  Montreal. 

All  these  types  of  dredges  require  dump-scows  to  receive  the 
dredged  material,  and  to  transport  it  to  the  designated  dumping 
grounds.  Occasionally  the  elevator  dredge  delivers  its  soil  by 
gravity  through  side  chutes. 

In  alluvial  rivers,  the  hydraulic-suction  dredge  has  been  used 
extensively  to  excavate  channels  through  the  bars.  The  main- 
tenance of  a  river  channel  by  dredging  requires  the  prompt  re- 
moval of  a  large  amount  of  material,  so  as  to  rapidly  concentrate 
the  flow  along  the  line  selected.  If  the  excavation  is  made  slowly, 
the  natural  forces  may  scour  a  channel  at  some  other  place  and 
the  tendency  to  fill  on  the  line  to  be  dredged  may  become  so  great 
as  to  cause  a  more  rapid  deposit  than  the  dredge  can  remove. 
Dredges  of  large  capacity  operated  continuously  are  therefore  re- 
quired, and  the  material  removed  must  be  deposited  at  such  a 
distance  from  the  channel  that  it  cannot  obstruct  it.  The  hy- 
draulic-suction dredge  (5)  discharging  through  a  pipe  line  best 
fulfills  these  conditions.  It  is  particularly  effective  if  the  spoil 
can  be  deposited  between  dikes,  as  is  usually  the  case  in  rivers 
which  are  being  regulated.  If  the  material  to  be  removed  is 
sand,  water  forced  through  jets  against  the  sand-bank  affords 
the  best  means  of  dislodging  it  and  moving  it  to  the  mouth  of  the 
suction  pipe.  If  the  material  is  clay,  some  form  of  cutter  is 
necessary.  Whether  a  dredge  should  be  self-propelling  or  should 
be  towed  from  place  to  place  by  a  towboat,  depends  on  the  fre- 
quency with  which  it  has  to  be  moved  and  the  distance  it  has  to 
travel.  A  tender  is  necessary  to  furnish  fuel  and  other  supplies, 
and  can  be  utilized  for  towing.  On  the  lower  Mississippi  river, 
a  small  tender  suitable  also  for  surveying  and  a  self-propelling 
dredge  makes  the  most  economical  combination. 

On  bars  at  the  mouths  of  rivers,  the  hopper  dredge  (4)  is  substi- 
tuted because  the  pipe-line  cannot  resist  wave  action  success- 
fully. This  is  a  self-propelling  suction  dredge  with  bins  in  which 
the  spoil  is  deposited.  When  filled,  the  dredge  is  moved  to  deep 
water  where  the  hoppers  are  emptied.  There  are  several  types 
of  hopper  dredges  depending  on  the  form  of  the  suction  head. 
Their  relative  economy  is  a  function  of  the  kind  of  material  to  be 
removed. 

Numerous  substitutes  for  the  hydraulic  dredge  have  been  pro- 


OBSTRUCTIONS  93 

posed.  While  most  of  them  are  so  visionary  as  not  to  merit 
mention,  a  few  of  them  can  be  utilized  in  cases  of  emergency, 
though  they  are  uneconomical  as  a  regular  means  of  channel 
improvement.  The  increased  velocity  which  the  paddle  wheels 
or  the  propeller  of  a  steamboat  imparts  to  the  water  can  be  uti- 
lized to  form  a  narrow  channel  of  the  width  of  the  boat  across  a 
bar,  and  the  stern  wheels  of  western  river  packets  have  frequently 
been  used  for  this  purpose.  On  the  upper  Rhine  a  washing 
dredge  has  been  devised  based  on  this  principle.  The  material 
removed,  however,  is  deposited  in  the  channel  a  short  distance 
below  and  has  to  be  rehandled  several  times,  while  a  suction  dredge 
removes  it  from  the  channel  in  one  operation. 

The  Long  scraper  (3)  was  used  quite  extensively  on  the  upper 
Mississippi  River  to  temporarily  deepen  the  channels  over  bars 
before  dredges  were  available.  It  consisted  of  a  triangular  frame 
of  oak  timber  with  buckets  or  cutters  of  boiler  iron  bolted  to  the 
lower  side.  It  was  attached  by  bolts  to  the  sides  of  the  boat, 
and  was  raised  and  lowered  by  ropes  controlled  by  steam  capstans. 
The  method  of  operating  the  scraper  was  to  move  the  boat  to 
the  head  of  the  bar  to  be  removed,  and  to  lower  the  scraper. 
The  wheels  of  the  steamer  were  then  backed  so  as  to  drag  the 
scraper  over  the  bar,  the  boat  floating  with  its  stern  downstream. 
The  scraper  was  then  raised  and  the  boat  returned  to  the  initial 
point,  and  the  operation  was  repeated  until  the  desired  depth  was 
obtained.  The  buckets  cut  up  and  loosened  the  material  on  the 
bar  and  then  conveyed  it  to  deep  water,  being  assisted  in  the 
movement  of  material  by  the  river  currents.  The  amount  of 
material  the  drag  could  remove  at  each  operation  was  quite 
limited,  and  the  time  occupied  by  the  boat  in  returning  for  its 
load  was  quite  long.  It  can  be  readily  appreciated  that  a  dredge 
working  continuously  will  excavate  a  channel  more  economically. 

It  also  has  been  proposed  to  utilize  the  river  currents  to  scour 
through  the  bars,  by  constructing  temporary  spur-dikes  either  by 
sinking  barges  on  the  line  proposed  which  can  afterwards  be 
pumped  out,  or  by  anchoring  them  by  spuds  and  cutting  off  the 
flow  under  them  by  shutters,  which  can  be  raised  after  they  have 
accomplished  the  work  for  which  they  were  intended.  Devices 
of  this  class  can  be  constructed  which  will  be  effective,  but  an 
analysis  of  the  cost  usually  will  show  that  either  a  permanent 
dike  or  dredging  is  more  economical. 


94  RIVERS  AND   HARBORS 


ROCK    EXCAVATIONS 

When  a  large  area  of  rock  of  considerable  thickness  is  to  be 
removed,  the  cheapest  method  of  doing  it  is  to  surround  the  area 
by  a  cofferdam,  pump  out  the  inclosure,  and  excavate  the  rock 
in  the  dry,  provided  the  cofferdam  does  not  interfere  seriously 
with  existing  navigation.  This  method  has  been  employed  ex- 
tensively for  the  rock  excavation  of  the  Rock  Island  Rapids  (3)  of 
the  Mississippi  River,  of  the  West  Neebish  cut  of  St.  Mary's 
river,  and  of  the  Livingston  Channel  of  the  Detroit  River  (2). 
The  rock  is  broken  up  by  the  ordinary  methods  of  rock  excavation 
employed  on  land,  and  is  removed  by  steam  shovel  and  dump 
cars,  or  by  cableways.  The  relative  economy  of  the  different 
methods  is  discussed  in  the  report  upon  the  excavation  of  the 
Chicago  Drainage  Canal  (6),  which  is  one  of  the  largest  works  of 
this  character  ever  undertaken. 

When  the  rock  obstruction  is  of  less  extent,  or  when  the  in- 
terests of  navigation  prevent  the  construction  of  a  cofferdam, 
several  methods  of  rock  removal  have  been  employed.  When 
the  rock  projects  above  the  river  bed  in  small  patches,  or  is  strati- 
fied so  as  to  be  broken  into  pieces  of  considerable  size,  the  Lob- 
nit  z  rock  crusher  is  an  economical  machine  for  rock  removal.  It 
consists  of  a  heavy  steel  plunger  which  is  allowed  to  fall  from  a 
considerable  height  and  breaks  up  the  rock  by  impact.  It  has 
been  used  extensively  for  that  purpose  in  the  rapids  of  the  Rhine 
(7),  and  in  those  of  the  Danube  (8).  When  the  rock  is  homo- 
geneous, this  method  is  not  so  successful.  The  first  blow  of  the 
crusher  breaks  up  a  certain  amount  of  material  which  remains 
in  place  and  the  succeeding  blows  merely  pulverize  the  broken 
fragments. 

On  the  upper  Rhine  (7)  the  diving  bell  was  used  extensively  in 
drilling  the  holes  and  in  inserting  the  charges  for  blasting  the 
rock.  In  the  United  States,  it  has  been  found  more  economical 
to  place  the  drills  on  barges  which  are  held  securely  in  position 
over  the  rock  to  be  excavated  by  means  of  spuds.  Such  drill 
barges  have  been  employed  on  portions  of  the  Rock  Island  rap- 
ids (3),  the  Middle  Neebish  Channel  of  St.  Mary's  River,  and 
the  Amherstburg  Channel  of  the  Detroit  River.  On  the  Danube 
also  drills  were  mounted  on  barges. 

In  the  Hell  Gate  (9)  of  the  East  River  of  New  York  Harbor, 


OBSTRUCTIONS  95 

the  method  of  rock  removal  was  to  sink  a  shaft  on  the  land  ad- 
joining the  section  of  channel  to  be  improved  and  run  a  tunnel 
under  the  channel.  An  amount  of  rock  was  then  removed  from 
the  tunnel  and  numerous  branches,  so  that  when  the  roof  of  the 
tunnel  was  allowed  to  fall  into  the  space  excavated,  a  sufficient 
channel  depth  would  be  created.  The  blasting  of  the  roof,  how- 
ever, caused  an  irregular  settlement  of  the  mass,  and  considerable 
rock  had  to  be  removed  by  operations  from  the  water  surface  be- 
fore the  required  depth  was  obtained  over  the  entire  area.  The 
later  removal  of  Blossom  Rock  (10)  from  San  Francisco  Harbor 
by  similar  means  was  more  successful. 

REMOVAL   OF   SNAGS 

On  account  of  the  caving  of  banks,  numerous  trees  fall  into  a 
river  and  lodge  on  sand  bars,  where  they  form  snags  dangerous 
to  navigation.  For  their  removal  a  snag-boat  is  required. 

On  the  lower  Mississippi,  the  snag-boats  are  powerful  machines, 
capable  of  removing  not  only  snags,  but  also  the  wrecks  of  vessels. 
Not  infrequently  they  have  had  to  replace  on  the  bank,  railway 
locomotives  that  have  fallen  into  the  river  at  the  inclines  to  car 
ferries.  On  the  smaller  tributaries,  a  strong  pair  of  shears  placed 
on  a  barge  is  utilized  for  snagging  purposes.  Early  in  the  season 
it  is  towed  to  the  upper  limits  of  the  snagging  district,  and  then  is 
allowed  to  float  downstream  with  the  current  as  the  work  progresses. 
A  motor  boat  usually  accompanies  such  a  barge  as  a  tender. 

An  important  function  of  the  snag-boat  is  the  cutting  of  trees 
on  the  edges  of  caving  banks  and  thus  preventing  them  from 
becoming  snags.  It  is  used  also  for  the  removal  of  other  ob- 
structions, such  as  wrecks  and  boulders. 

In  certain  rivers  of  the  United  States  snags  have  been  de- 
posited in  such  quantities  as  not  only  to  endanger  navigation 
but  also  to  interfere  seriously  with  the  discharge  of  the  rivers. 
On  the  Red  and  Atchafalaya  rivers  these  accumulations  of  snags, 
termed  rafts,  obstructed  the  channel  for  many  miles,  and  the  im- 
provement of  these  rivers  has  consisted  in  the  removal  of  these 
rafts.  Their  removal  in  the  Atchafalaya,  which  is  an  outlet  to 
the  Mississippi,  has  increased  its  flood  discharge  from  less  than 
100,000  second-feet  to  over  400,000  second-feet,  and  has  rendered 
necessary  the  construction  of  submerged  sills  near  its  junction 
with  the  Mississippi  to  prevent  a  further  enlargement. 


96  RIVERS  AND   HARBORS 

On  the  Red  River  (11)  the  removal  of  the  rafts  has  been  ac- 
companied by  a  marked  lowering  of  the  river  bed,  in  some  places 
of  about  25  feet.  When  the  rafts  existed,  they  acted  as  dams  and 
caused  an  immense  deposit  of  sediment,  which  affected  the  river 
slopes  for  many  miles  above  them.  The  river  discharge,  unable 
to  follow  the  main  channel,  escaped  through  numerous  sloughs 
excavated  through  the  river  banks,  some  of  which  afforded  a  pre- 
carious navigation  during  high  stages.  When  the  rafts  were  re- 
moved, these  sloughs  were  closed  and  the  river  flow  concentrated 
in  a  single  channel,  which  has  adjusted  its  slope  to  the  changed 
conditions.  The  concentrated  flow  also  has  increased  bank  cav- 
ing in  certain  sections  of  the  river,  and  numerous  cut-offs  have 
resulted.  Above  these  sections  the  bed  of  the  river  has  been 
lowered  about  five  feet.  Below  them  it  has  been  raised  about 
3J  feet.  Levees  have  also  been  constructed  along  the  river 
banks,  and  these  changes  in  the  regimen  of  the  river  have  some- 
times been  ascribed  erroneously  to  the  levee  construction. 

BUOYS,   LIGHTS  AND   BEACONS 

An  important  aid  to  navigation  consists  in  so  marking  the  channel 
that  the  pilot  can  follow  it  readily  by  day  or  night.  Buoys, 
beacons  and  lights  are  used  for  this  purpose.  In  the  United 
States  the  Bureau  of  Lighthouses  has  charge  of  their  maintenance, 
but  an  intimate  cooperation  is  necessary  between  this  bureau  and 
those  engaged  in  channel  improvement.  On  the  western  rivers 
this  cooperation  is  insured  by  assigning  the  duties  of  Lighthouse 
Superintendent  to  a  Division  or  District  Engineer.  While  per- 
forming this  function,  he  reports  directly  to  the  Chief  of  the 
Lighthouse  Bureau.  An  engineer  officer  is  detailed  also  for  con- 
sultation or  to  superintend  the  construction  and  repair  of  any  aids 
to  navigation  authorized  by  Congress,  in  the  other  lighthouse 
districts. 

One  method  of  marking  the  channel  is  by  means  of  buoys 
showing  its  limits;  another  is  by  a  system  of  ranges  which  locate 
its  center  line.  These  two  methods  are  frequently  combined. 
On  the  Mississippi  River  the  limits  of  a  dredged  channel  are  usually 
marked  by  four  buoys  placed  by  the  dredge  employees  when  the 
cut  is  made,  and  the  center  line  is  marked  by  two  white  posts  on  the 
opposite  banks  of  the  river  on  which  lanterns  are  maintained  at 


OBSTRUCTIONS  97 

night  by  the  lighthouse  establishment.  Lights  are  also  main- 
tained in  the  river  bends  at  such  distances  that  at  least  one  is 
always  in  view  in  advance  of  the  boat,  whether  moving  upstream 
or  downstream.  As  the  channel  is  frequently  changing  and  the 
banks  are  unstable,  a  cheap  structure  that  can  be  readily  moved 
is  necessary. 

Since  the  channels  connecting  the  Great  Lakes  are  stable,  the 
range  lights  are  placed  in  lighthouses.  Two  lights  are  placed  on 
the  same  side  of  the  river,  on  the  prolongation  of  the  center  line, 
and  frequently  one  of  them  —  a  low  structure  —  is  located  in 
shoal  water,  while  the  rear  light  is  on  the  river  bank  at  a  greater 
elevation.  When  the  boat  is  in  the  channel,  the  two  lights  ap- 
pear one  vertically  above  the  other.  On  channels  of  consider- 
able length,  range  lights  may  be  established  at  both  extremities. 
Lights  frequently  are  placed  on  the  buoys;  the  New  York  State 
Barge  Canal,  recently  completed,  is  thus  lighted. 

On  the  seacoast  and  on  the  Great  Lakes,  lighthouses  (12)  are 
established  at  frequent  intervals  to  warn  the  mariner  of  the  prox- 
imity of  land  or  of  dangerous  shoals.  The  general  project  con- 
templates ultimately  surrounding  the  United  States  with  a  sys- 
tem of  lighthouses  whose  areas  of  visibility  intersect,  so  that 
the  light  from  at  least  one  will  be  visible  on  a  clear  night  before 
the  vessel  arrives  within  dangerous  proximity  to  the  shore.  The 
mariner  can  also  determine  his  location  with  considerable  accu- 
racy, when  the  light  first  appears  above  the  horizon,  as  every 
lighthouse  has  its  characteristic  light,  and  the  height  of  the  light 
above  sea  level  is  published.  The  lights  usually  are  differentiated 
by  means  of  revolving  shutters  which  obscure  the  light  for  a  certain 
length  of  time.  By  varying  the  length  of  the  eclipses  and  flashes, 
a  large  variety  of  clearly  distinguishable  characteristics  can  be 
obtained  for  the  different  lights. 

The  form  of  lens  and  reflector  for  lighthouses  and  lanterns 
which  will  throw  the  maximum  volume  of  light  in  the  direction 
desired,  has  been  studied  carefully.  The  construction  of  light- 
houses in  exposed  localities  is  one  of  the  most  difficult  of  engineer- 
ing feats.  Other  auxiliary  aids  are  fog  sirens  and  whistling  buoys, 
to  warn  the  mariner  of  danger  during  fogs.  They  are  rarely 
employed  on  rivers  except  at  their  mouths  (13). 


CHAPTER  X 

RESERVOIRS  AND  LEVEES  AS  A  MEANS   OF  IMPROVING 
NAVIGATION 

RESERVOIRS 

The  advocates  of  the  improvement  of  the  low-water  navigation 
of  rivers  by  means  of  reservoirs  (1)  usually  assume  that  a  river's 
depth  has  the  same  relation  to  its  discharge  as  that  which  exists 
in  a  conduit  or  sewer,  and  that  by  increasing  the  discharge  there 
will  be  a  similar  increase  in  the  depth  of  the  navigable  channel. 
This  can  be  true  only  when  the  bed  is  immobile. 

As  explained  in  preceding  chapters,  during  extreme  low  stages 
the  velocity  is  small  in  the  pools  and  in  the  upper  reaches  of  a 
river,  and  is  insufficient  to  cause  bank  caving.  As  the  discharge 
is  increased  the  velocity  in  the  pools  increases  more  rapidly  than 
over  the  bars  and  it  soon  has  sufficient  force  to  scour  its  bed.  The 
material  thus  removed  from  a  pool  is  deposited  to  a  great  extent 
on  the  bar  below,  raising  its  crest.  It  was  pointed  out  also  that 
in  a  river  similar  to  the  Mississippi,  under  certain  conditions  the 
rise  in  bar  height  may  equal  one-half  the  increase  in  stage,  but  it- 
does  not  follow  therefrom  that  a  permanent  increase  in  the  low- 
water  discharge  would  be  accompanied  by  an  increase  in  depth 
over  the  bars  of  one-half  that  usually  computed.  The  reason 
that  the  crest  of  a  bar  on  a  rising  river  does  not  attain  a  greater 
height  is  that  the  elevation  of  the  river  bed  is  a  much  slower 
process  than  the  change  in  stage.  The  crest  of  a  flood  passes 
before  it  has  had  an  opportunity  to  produce  its  maximum  effect, 
and  a  falling  river  causes  a  scour. 

If  the  discharge  remains  constant  at  a  given  stage,  the  eleva- 
tion of  the  bar  continues  until  a  dam  is  formed  at  the  lower  end 
of  the  pool.  This  reduces  the  velocity  over  it  sufficiently  to  pre- 
vent caving,  or  the  ratio  of  the  velocities  in  the  pool  and  over 
the  bar  becomes  such  that  the  material  scoured  in  the  pool  can  be 
transported  across  the  bar  as  fast  as  it  tends  to  deposit.  The 
effect  of  permanently  increasing  the  low-water  discharge  in  a  river 

98 


RESERVOIRS   AND   LEVEES  99 

therefore  is  to  increase  largely  the  depth  in  the  pools  and  to  im- 
prove but  slightly  the  channels  over  the  bars.  For  example,  the 
natural  low-water  discharge  of  the  Mississippi  River  at  St.  Paul  is 
about  2500  second-feet,  its  extreme  flood  discharge  is  about  100,000 
second-feet,  and  its  slope  is  about  0.4  foot  per  mile.  At  low  stages 
the  depth  of  the  river  unimproved  by  regulation  was  about  10 
feet  in  pools,  and  about  1  foot  on  some  of  the  bars.  As  the  low- 
water  discharge  of  the  river  increases  by  the  inflow  of  tributaries, 
the  channel  depths  progressively  increase.  When  a  low-water 
discharge  equal  to  the  extreme  flood  discharge  at  St.  Paul  is  at- 
tained by  natural  causes  with  the  same  slope  of  0.4  foot  per  mile, 
the  depth  in  pools  becomes  approximately  100  feet,  but  the  natu- 
ral channel  depths  over  some  bars  is  about  4  feet  at  low  stages. 
A  channel  depth  of  9  feet,  which  is  exceeded  on  the  upper  river 
during  floods,  can  be  maintained  on  the  lower  river  for  the  same 
discharge  only  by  dredging.  On  the  other  hand,  variations  in 
discharge  in  an  immobile  bed  produce  variations  in  depths  which 
correspond  to  those  computed  for  conduits.  This  occurs  in  the 
rock  cuts  of  the  Neebish  channels  in  the  St.  Mary's  River,  where 
the  variations  are  caused  by  the  fluctuations  of  Lake  Superior. 

The  attempt  has  been  made  to  improve  the  navigation  of  the 
Mississippi  by  reservoirs  (1).  Six  reservoirs  have  been  constructed 
at  its  headwaters  capable  of  impounding  about  97,000,000,000 
cubic  feet  of  water,  which  it  was  estimated  would  maintain  a  low- 
water  discharge  of  5000  second-feet  at  St.  Paul.  The  original 
project  contemplated  the  construction  of  41  reservoirs  on  the 
tributaries  of  the  river  in  the  states  of  Minnesota  and  Wisconsin, 
but  the  results  attained  by  those  built  have  not  justified  a  further 
extension  of  the  system. 

It  is  estimated  that  the  increase  in  the  low-water  discharge 
which  the  reservoirs  are  capable  of  producing  will  cause  an  increase 
in  low-water  navigable  depths  at  St.  Paul  of  2  feet,  and  an  appreci- 
able increase  in  depth  from  St.  Paul  to  Lake  Pepin,  a  distance  of 
52  miles.  This  portion  of  the  river  has  been  improved  by  regula- 
tion, however,  its  banks  have  been  protected  from  caving,  and  the 
channel  across  bars  has  been  given  such  a  direction  as  to  convert 
bad  crossings  into  good  ones.  The  increased  discharge  is  therefore 
confined  to  a  channel  in  which  its  injurious  effects  have  been 
limited. 

Below  Lake  Pepin,  where  the  improvement  is  not  so  far  ad- 


100  RIVERS  AND   HARBORS 

vanced,  the  increase  in  the  low-water  discharge  has  produced  no 
appreciable  increase  in  navigable  depths.  This  is  due  to  a  more 
potent  cause  than  that  mentioned  above.  The  reservoir  capacity 
of  Lake  Pepin  is  as  efficient  a  regulator  of  the  low-water  discharge 
of  the  river  as  are  the  artificial  reservoirs  constructed  at  its  head- 
waters; and  before  their  construction  it  caused  a  minimum  dis- 
charge below  it  equal  to  that  which  they  are  now  capable  of  main- 
taining at  St.  Paul. 

When  the  channel  has  been  improved  by  regulation  and  it  is 
necessary  to  obtain  greater  navigation  depths  than  this  method  of 
improvement  will  afford,  the  result  can  be  attained  by  increasing 
the  low-water  discharge  by  means  of  reservoirs.  As  a  substitute 
for  regulation,  however,  the  results  attained  do  not  justify  the 
construction  of  these  reservoirs,  even  under  such  favorable  con- 
ditions as  those  which  exist  at  the  headwaters  of  the  Mississippi, 
where  large  volumes  of  water  were  impounded  at  an  exceedingly 
low  cost  for  the  dams  (2). 

The  practical  manipulation  of  the  reservoirs  at  the  headwaters 
of  the  Mississippi  presents  difficulties  which,  while  not  insurmount- 
able, mitigate  against  their  use  for  the  purpose  intended.  The 
original  act  of  Congress  authorizing  their  construction  stated  that 
the  purpose  was  to  improve  the  navigation  of  the  river  below  St. 
Paul.  There  are  numerous  other  interests  than  the  navigation  of 
the  lower  river  which  have  to  be  considered.  The  fall  in  the  river 
between  the  headwaters  and  St.  Paul  has  been  utilized  in  several 
localities  for  the  production  of  water  power,  and  the  water  power 
companies  desire  to  preserve  a  uniform  flow  at  all  times.  The 
closing  of  the  gates  in  the  dams  to  conserve  the  water  at  certain 
seasons  causes  a  fluctuation  in  flow  detrimental  to  their  interests. 
The  logging  companies  require  a  certain  amount  of  water  to  float 
their  logs  in  the  tributaries  which  are  the  outlets  to  the  reservoirs. 
If  too  little  water  is  allowed  to  pass  the  dams  the  logs  ground  on 
the  river  bars.  If  too  much  water  is  released,  the  bottom  lands 
are  overflowed  and  many  of  the  logs  lodge  upon  them.  Also, 
the  farmer  who  utilizes  the  bottom  lands  to  produce  crops  of  hay 
strenuously  objects  to  any  manipulation  of  the  gates  which  will 
flood  the  land,  particularly  at  a  time  when  he  is  harvesting  his 
crop.  This  is  usually  the  time  when  the  greatest  discharge  from 
the  reservoirs  is  required  to  maintain  the  proper  river  heights. 

The  navigation  interests  above  St.  Paul  have  also  to  be  con- 


RESERVOIRS  AND   LEVEES  ;lOl' 

sidered,  and  there  is  a  demand  for  the  utilization  of  the  reservoirs 
to  reduce  flood  heights,  but  those  living  below  the  dams  desire  the 
closure  of  the  gates  during  floods,  while  those  above  them  protest 
against  the  flooding  of  the  land  by  back-water  caused  by  filling 
the  lakes.  In  addition  to  the  difficulties  arising  from  human 
agencies,  nature  adds  to  the  complexity  of  the  problem  by  the 
variations  in  rainfall.  In  some  years  not  only  does  a  drought  tax 
the  reservoirs  to  their  capacity,  but  there  is  not  enough  rain  during 
the  flood  season  to  fill  them  again,  and  a  second  low-water  season 
occurs  without  a  sufficient  amount  of  water  stored  for  their  proper 
functioning. 

A  period  of  about  a  month  formerly  elapsed  before  the  water 
allowed  to  escape  from  Lake  Winnibigoshish  produced  an  appre- 
ciable effect  on  the  river  heights  at  St.  Paul,  and  it  has  sometimes 
happened  that  in  attempting  to  harmonize  the  conflicting  interests 
there  has  been  too  great  a  delay  in  opening  the  valves  at  the  dams, 
and  the  reservoirs,  instead  of  increasing  the  extreme  low-water 
discharge  at  St.  Paul  by  the  amount  proposed,  have  materially 
reduced  it.  The  gage  at  St.  Paul  quite  frequently  has  had  a  read- 
ing lower  by  about  one  foot  since  the  reservoirs  have  been  built 
than  was  on  record  prior  to  their  construction,  but  the  manipula- 
tion of  the  dams  is  now  so  regulated  that  it  is  exceptional  for  these 
low  readings  to  occur  during  the  period  of  navigation. 

By  straightening  the  upper  river,  the  time  required  for  the 
transmission  of  the  discharge  has  been  diminished  and  the  follow- 
ing general  rules  are  now  observed  in  operating  the  reservoirs: 

"  (a)  The  discharge  must  not,  by  operation  of  the  reservoirs,  be 
reduced  below  the  normal  low- water  flow  of  the  streams  affected. 
This  rule  is  necessary  in  the  interest  of  manufacturers. 

"(b)  When  logs  arrive  in  the  reservoirs,  they  must  be  sluiced 
through.  Transportation  of  logs  by  floating  is  a  form  of  com- 
merce, and  the  main  form  of  commerce  on  the  streams  affected  by 
the  reservoirs.  It  is  dangerous  to  the  dams  to  allow  accumulations 
of  logs,  so  that  they  must  be  sluiced  through  even  in  times  of  flood. 

"  (c)  The  winter  flow  is  so  regulated  as  to  make  room  for  39 
billion  cubic  feet  of  water  at  the  end  of  winter.  This  is  the 
amount  ordinarily  to  be  expected  in  the  spring  floods. 

"(d)  From  the  spring  thaw  until  the  dry  season  of  summer 
(ordinarily  until  about  July  10)  as  much  water  is  retained  in  the 
reservoirs  as  possible,  subject  to  rules  (a)  and  (b). 


,-;.l62  RIVERS  AND  HARBORS 

"(e)  When  the  gage  at  St.  Paul  has  fallen  nearly  to  3  feet, 
water  is  released  so  as  to  keep  the  gage  at  this  reading.  If  there  is 
not  enough  water  for  this  purpose,  then  the  greatest  constant 
depth  possible  is  maintained. 

"(f)  When  during  the  low-water  stage,  there  is  not  sufficient 
depth  for  the  steamer  plying  between  Aitken  and  Grand  Rapids, 
and  the  quantity  of  water  in  the  reservoirs  is  sufficient,  enough 
water  is  released  on  request  to  make  the  trip  possible.  This  use  of 
the  reservoirs  is  occasional  "(3). 

There  are  numerous  other  localities  where  reservoirs  have  been 
constructed  and  where  the  water  has  been  used  incidentally  to 
increase  the  flow  during  low  water.  With  the  exception  of  those 
at  the  headwaters  of  the  Volga,  their  primary  purpose  has  been 
either  to  create  water  power  or  to  reduce  the  dangers  from  floods. 
One  of  the  greatest  enterprises  of  this  kind  is  the  reservoir  con- 
struction being  undertaken  on  the  Ottawa  River  and  its  tributaries 
in  Canada. 


IMPROVING   THE   LOW-WATER   CHANNEL   BY   CONFINING   THE   FLOOD 
DISCHARGE   BETWEEN   LEVEES 

The  question  of  the  influence  of  levees  on  the  low-water  channel 
of  a  river  has  been  a  subject  of  discussion  for  many  years,  particu- 
larly in  the  Mississippi  Valley  (4).  During  floods  a  river  usually 
has  a  discharge  from  twenty  to  fifty  times  that  of  low  water,  and 
the  water  moves  in  the  channel  with  at  least  twice  the  low-stage 
velocity.  Hence  its  energy,  which  is  a  measure  of  its  scouring 
effect,  is  from  eighty  to  two  hundred  times  as  great  during  floods 
as  during  low  stages. 

A  great  deal  of  this  energy  is  dissipated  in  the  water  which  flows 
over  the  banks  and  floods  the  alluvial  valley.  It  is  claimed  that  if 
this  water  is  prevented  from  escaping  from  the  river  bed  by  the 
construction  of  levees,  the  force  acting  during  floods  will  be  greatly 
augmented,  and  that  it  will  produce  a  powerful  scouring  effect 
on  the  low-water  channel. 

That  levees  largely  increase  the  force  acting  during  floods  is 
unquestionable.  On  the  lower  Mississippi  from  Cairo  to  Vicks- 
burg  a  confined  flood  of  2,000,000  second-feet  can  now  be  carried, 
while  prior  to  the  construction  of  levees,  the  maximum  discharge 
at  Vicksburg  was  about  1,000,000  second-feet.  It  is  a  serious 


RESERVOIRS   AND   LEVEES  103 

question,  however,  whether  or  not  this  confined  force  is  producing 
useful  work.  For  such  a  purpose  it  is  necessary  not  only  to  in- 
crease the  force  but  also  to  give  it  the  proper  direction.  By  in- 
creasing the  intensity  of  the  fire  under  a  steam  boiler  a  greater 
force  is  generated,  but  unless  a  proper  direction  is  given  to  it 
through  a  steam  pipe  leading  to  a  steam  engine,  the  increased 
force,  instead  of  producing  useful  work,  may  become  one  of  de- 
struction, and  burst  the  boiler  shell.  While  not  conclusive,  there 
is  presumptive  evidence  that  levee  construction  has  enlarged  the 
bed  of  the  Mississippi  (5) .  If  the  river  followed  the  same  channel 
during  high  and  low  stages,  as  explained  in  the  discussion  of  the 
effect  of  reservoirs  on  channel  depths,  an  increase  in  the  discharge 
would  cause  an  increased  caving  in  the  bends  and  a  raising  of  the 
crests  of  bars,  deepening  the  river  in  the  pools  to  the  detriment  of 
the  crossings. 

It  is  impossible,  however,  to  construct  levees  so  as  to  force  the 
flood  discharge  to  follow  the  low-water  channel.  If  levees  were 
constructed  with  this  object  in  view  they  would  have  to  be  placed 
so  close  to  the  banks  as  to  be  destroyed  by  caving.  When  lo- 
cated from  half  a  mile  to  ten  miles  from  the  river  bank,  as  is  the 
case  on  the  lower  Mississippi,  the  river  during  flood  stages  follows 
a  course  far  different  from  that  which  it  has  during  low  water. 
While  at  some  localities  the  scour  during  high  and  low  water  may 
coincide,  at  others  the  scour  during  floods  may  be  on  bars  above 
low  water  and  a  deposit  may  occur  in  the  low-water  channel. 

Whether  the  resultant  combination  of  scour  and  fill  during 
flood  stages  increases  or  diminishes  the  amount  of  work  which  it 
is  necessary  for  the  low- water  discharge  to  do  in  order  to  produce 
a  certain  channel-depth  is  a  question  of  fact  difficult  to  determine. 
On  the  lower  Mississippi,  a  certain  amount  of  dredging  is  re- 
quired to  provide  a  channel  depth  of  nine  feet  at  low  water;  but 
whether  or  not  the  amount  of  dredging  required  to  obtain  that 
result  has  been  increased  or  diminished  by  levee  construction  is 
unknown. 

The  low-water  channel  can  be  maintained  readily  by  dredging, 
however,  and  since  Congress  has  authorized  the  construction  of 
levees  for  flood  protection,  for  which  purpose  they  are  essential, 
their  employment  for  improving  low-water  conditions  has  ceased 
to  be  a  question  of  practical  importance  on  this  river,  and  it  may 
be  relegated  to  the  zone  of  academic  discussion. 


104  RIVERS  AND   HARBORS 

The  theory  that  levees  would  improve  navigation  was  the 
natural  sequence  to  the  theory  formerly  held  by  many  advocates 
of  river  regulation  that  the  proper  method  for  improving  a  river 
was  to  straighten  it  and  give  it  a  uniform  slope  and  depth.  If  it 
were  practicable  to  so  improve  the  low-water  channel  of  a  river,  it 
logically  follows  that  an  increase  in  discharge  would  produce  an 
enlargement  of  the  low-water  section.  But  when  it  has  been  dem- 
onstrated that  it  is  impossible  to  eradicate  the  curves  from  the 
low-water  channel,  that  the  slopes  in  pools  and  on  crossings  must 
be  preserved,  and  that  a  variable  depth  in  bends  and  bars  must 
exist,  the  corollary  suffers  the  same  fate  as  the  proposition  on 
which  it  is  founded. 

The  present  practice  in  European  rivers  is  to  ignore  the  high- 
water  discharge  in  improving  the  low-water  channel  except  to 
prevent  its  injurious  effects,  and  to  allow  it  to  escape  with  as 
great  freedom  as  possible.  For  this  purpose,  even  the  works  of 
improvement  in  the  river  bed  are  given  as  low  an  elevation  as 
practicable. 

The  original  project  for  the  improvement  of  the  Danube  (6), 
adopted  in  1882,  contemplated  restraining  its  floods  by  means  of 
levees  and  concentrating  its  flow  in  a  single  channel  between  two 
fixed  banks,  relying  on  flood  control  to  ameliorate  low-water  con- 
ditions. The  cut  in  front  of  the  city  of  Vienna  executed  from  1870 
to  1875  illustrates  the  application  of  the  general  principles.  The 
river  was  confined  here  to  a  single  channel,  nearly  straight,  484.5 
meters  wide  at  mid-stage  (Nullwasser) ,  and  760  meters  wide  at 
flood  stage.  The  enlargement  of  the  low-water  bed  was  on  the 
bank  opposite  the  city  and  at  an  elevation  of  1.5  meters  above 
Nullwasser.  From  this  section  all  trees  and  shrubbery  were 
removed.  The  low-water  bed  had  a  discharge  of  1700  cubic 
meters  at  a  zero  stage  and  only  600  to  700  cubic  meters  at  low 
water,  which  produced  a  winding  and  variable  channel  along  the 
Vienna  water  front,  where  it  was  desirable  to  permit  access  to  the 
banks  for  the  greatest  possible  length. 

A  further  contraction  of  the  low-water  channel  then  became 
necessary.  This  was  accomplished  by  means  of  a  system  of  spur- 
dikes  extending  98  meters  from  the  left  bank,  with  an  elevation 
of  2.5  meters  below  Nullwasser  at  the  bank,  and  a  slope  of  about 
1.5  per  cent.  These  dikes  incline  upstream,  making  an  angle  of 
about  75°  with  the  axis  of  the  bed,  and  they  are  generally  about 


RESERVOIRS   AND   LEVEES  105 

100  meters  apart.     At  some  places  their  outer  ends  have  been 
connected  by  submerged  longitudinal  dikes. 

Notwithstanding  this  work  the  channel  is  still  sinuous,  but  a 
depth  of  4  meters  below  the  zero  of  the  gage  has  been  realized  at 
those  portions  of  the  Vienna  front  used  as  quays,  and  a  navigable 
channel  of  2.0  meters  has  been  obtained  at  extreme  low  water 
except  during  periods  of  freezing  weather,  when  all  navigation  is 
interrupted. 

On  other  portions  of  the  river  excessive  dredging  was  required  to 
maintain  the  low-water  channel,  and  a  similar  method  of  con- 
traction was  attempted  at  first.  In  1899  a  new  project  was  adopted, 
which  contemplated  a  low-water  channel  of  a  uniform  width  of 
210  meters  and  a  depth  of  3.5  meters.  This  channel  is  limited 
by  means  of  spur-dikes  inclined  upstream  at  an  angle  of  70° 
with  the  axis  of  the  bed.  They  have  an  elevation  of  mid-stage 
at  the  bank  and  a  slope  of  from  5°  to  10°  to  low  water  and  are 
there  extended  by  submerged  dikes  with  a  slope  of  3°. 

All  attempts  to  straighten  the  river  have  now  been  abandoned, 
and  the  channel  follows  a  series  of  curves,  so  that  the  centrifugal 
force  of  the  water  tends  to  secure  the  stability  of  the  bed.  At 
certain  localities  longitudinal  dikes  have  been  constructed  on  the 
concave  banks. 

In  a  comparison  of  the  improvement  of  the  Danube  with  that 
of  the  Rhone,  M.  Armand,  Chief  Engineer  of  the  Fonts  et  Chausse'es, 
makes  the  following  remarks  (6) : 

"It  is  known  that  works  of  improvement  of  rivers  with  a 
mobile  bed,  undertaken  by  the  method  of  contraction,  have 
often  led  to  mistakes.  The  contraction  of  the  bed  across  a 
shoal  increases  the  erosive  force  of  the  water,  the  shoal  is  deep- 
ened, but  the  water  plane  is  lowered  in  the  pool  above,  and  the 
upper  shoal  is  aggravated. 

"That  is  what  occurred  in  the  Rhone  following  the  first  works 
of  improvement  executed  before  1882,  and  that  is  what  led 
the  engineers  charged  with  the  improvement  of  that  river  to 
adopt  a  different  plan,  based  on  the  preservation  of  the  natural 
forms  of  the  bed,  and  its  division  into  successive  pools,  which 
it  is  sought  to  modify  as  little  as  possible,  except  when  it  is 
necessary  in  order  to  realize  at  low  water  the  draft  of  water 
desired. 

"The  dangers  of  the  method  of  contraction  have  not  escaped 


106  RIVERS  AND   HARBORS 

the  Engineers  of  the  Danube  Commission.  While  limiting  the 
river,  they  have  studied  a  judicious  trace  of  the  low-water  bed 
which  respects  the  natural  sinuosities  of  the  river  as  far  as 
possible,  and  by  the  use  of  submerged  works  avoids  too  great 
a  depth  at  some  points  of  the  low-water  bed.  The  method 
which  they  have  employed  is  in  the  main  intermediate  between 
simple  contraction  and  the  method  of  preservation  of  the 
natural  forms  applied  on  the  Rhone. 

"The  results  obtained  at  this  time  appear  to  show  that  they 
have  been  right;  and  these  results  have  been  obtained  by  works 
of  an  execution  assuredly  less  delicate  than  the  works  of  direc- 
tion necessary  on  the  Rhone. 

"Moreover,  the  conditions  of  the  two  rivers  were  appreciably 
different.  On  the  Danube  the  slope  is  less,  the  low-water  dis- 
charge is  two  or  three  times  as  great,  and  there  is  little  uneasi- 
ness that  extreme  low-water  will  be  produced  by  great  freezing 
when  navigation  is  interrupted  by  ice. 

"It  is  probable  that  the  bed  of  the  Danube  is  less  subject  to 
scour  than  that  of  the  Rhone,  or  that  it  presents  at  certain 
points  rocky  shoals  which  do  not  scour;  this  appears  to  result 
from  the  fact  that  the  great  cut  at  Vienna,  straightening  the 
river  for  a  great  length,  has  produced  only  a  very  small  lowering 
of  the  low-water  plane  above." 


CHAPTER  XI 

FLOOD  PROTECTION 

In  discussing  the  protection  of  land  from  the  ravages  of  flood 
waters,  it  is  necessary  to  bear  in  mind  the  divisions  of  a  river 
basin  into  an  area  of  erosion,  one  of  deposition,  and  one  where 
the  forces  of  erosion  and  deposition  are  in  unstable  equilibrium. 
The  problem  of  flood  protection  differs  materially  in  the  three 
sections. 

On  the  hillside  where  the  precipitation  tends  to  remove  the  soil, 
the  best  protection  is  a  forest  growth,  as  the  roots  of  the  trees 
bind  the  soil  together  and  offer  a  great  resistance  to  the  flow  of 
water  where  any  local  scour  starts  an  incipient  gulley.  The  re- 
moval of  the  forest  in  itself  is  not  necessarily  destructive,  as  the 
stumps  and  roots  that  remain  resist  decay  for  a  long  period  and  a 
second  growth  of  timber  can  replace  the  one  removed  before  serious 
erosion  results.  But  if  the  land  is  cleared  for  cultivation  great 
damage  may  be  caused.  The  water  flowing  down  the  hillside 
then  removes  the  humus  which  has  been  deposited  on  it  by  the 
leaves  falling  from  the  trees  through  ages  and  which  gives  it  the 
fertility  necessary  for  the  growth  of  vegetation. 

The  best  substitute  for  the  forest  is  a  growth  of  grass,  par- 
ticularly of  those  varieties  whose  roots  tend  to  grow  in  a  hori- 
zontal direction  near  the  surface  of  the  ground.  When  not  ex- 
posed to  freezing  weather,  Bermuda  grass  is  particularly  adapted 
for  this  purpose  and  is  used  extensively  in  southern  latitudes  for 
the  protection  of  the  slopes  of  levees  and  other  embankments. 
It  requires  considerable  time,  however,  to  form  a  good  bond  by 
the  interlacing  of  the  roots,  and  in  sandy  soils  there  is  a  liability 
of  wash  from  heavy  rains  scouring  the  bank  and  even  carrying 
away  the  tufts  of  grass  before  the  sod  is  formed. 

The  plowing  of  furrows. at  right  angles  to  the  slope  is  of  service 
during  light  rains,  but  it  is  of  little  use  during  the  heavy  storms, 
which  are  the  great  cause  of  the  damage  wrought. 

The  scour  can  largely  be  prevented  by  terracing  the  hillside  and 
guiding  the  rain  water  by  drains  across  the  terraces,  but  this  method 

107 


108  RIVERS  AND   HARBORS 

is  applicable  only  in  special  cases  on  account  of  its  excessive  cost. 
It  has  been  employed,  however,  in  thickly  settled  countries  where 
land  has  a  great  value,  and  particularly  where  irrigation  is  neces- 
sary for  agriculture,  the  formation  of  level  areas  being  essential 
to  the  proper  use  of  the  irrigating  waters  for  crops. 

In  the  area  of  deposition  the  problem  which  confronts  the  en- 
gineer is  usually  to  prevent  the  detritus  which  is  washed  down 
from  the  hills  from  spreading  over  the  bottom  lands  and  destroy- 
ing their  fertility.  For  this  purpose  levees  have  frequently  been 
constructed  which  limit  the  flow  to  a  narrow  channel  and  thus 
preserve  the  remainder  of  the  bottom  lands  from  the  action  of 
the  stream.  When  levees  are  thus  employed  a  fill  invariably 
occurs  by  deposition  at  the  point  where  the  steep  slope  down  the 
hillside  changes  to  a  gentle  slope  across  the  bottom  lands;  and,  to 
maintain  the  channel  within  the  limits  prescribed,  the  levees 
must  be  raised  from  time  to  tune,  or  the  material  deposited  must 
be  removed.  Around  Lake  Biwa  in  Japan,  where  levees  have 
been  used  for  this  purpose  for  generations,  they  are  of  inordinate 
height  on  insignificant  streams  whose  discharge  is  limited  to  the 
water  flowing  during  storms.  The  deposit,  however,  is  fre- 
quently a  mixture  of  small  cobble  stones  and  gravel  which  would 
make  a  good  material  for  road  construction,  and  in  a  country 
where  macadam  roads  prevail,  the  annual  removal  of  the  detritus 
may  be  an  economical  solution  of  the  problem. 

Another  method  of  protecting  the  bottom  lands  is  by  the  con- 
struction of  dams  across  the  ravines  in  which  the  stream  flows, 
above  its  entrance  into  the  valley,  thus  forming  pockets  in  which 
the  detritus  is  deposited.  These  pockets  rapidly  fill,  necessitating 
a  periodic  raising  of  the  dams  or  the  building  of  additional  ones. 
This  method  of  construction  has  been  employed  extensively  on 
the  tributaries  of  the  Sacramento  River  (1)  to  prevent  injury  to  the 
Sacramento  Valley  by  material  washed  from  the  hillsides  in  the 
hydraulic  mining  of  gold. 

In  hydraulic  mining,  the  material  of  a  hillside  is  washed  into 
sluices  by  jets  of  water  directed  against  it  under  heavy  pressure, 
and  the  waste  material  is  allowed  to  flow  into  the  neighboring 
streams  after  the  gold  has  been  extracted.  This  detritus  flowed 
into  the  tributaries  of  the  Sacramento  River  and  rapidly  destroyed 
the  equilibrium  between  scour  and  fill  which  nature  had  estab- 
lished through  the  work  of  geological  ages.  Large  sand  waves 


FLOOD   PROTECTION  109 

which  contained  none  of  the  elements  necessary  to  cause  the 
growth  of  vegetation,  and  which  destroyed  the  fertility  of  the  land 
over  which  they  spread,  were  placed  in  motion  down  the  main  river. 

These  sand  waves  have  a  motion  of  about  4  miles  per  year,  and 
not  only  injure  the  agricultural  interests  by  killing  vegetation  as 
they  pass  over  the  land,  but  their  crests  fill  the  river  bed  and  cause 
increased  flood  height  at  the  localities  they  successively  occupy 
in  their  passage  downstream.  They  also  affect  navigation  in- 
juriously. By  this  motion  of  sand,  nature  is  attempting  to  read- 
just the  relations  between  scour  and  fill  which  man  has  disturbed. 

Even  if  no  further  deterioration  of  the  river  channel  by  the  min- 
ing interests  is  permitted,  it  will  require  a  long  period  of  time  for 
the  river  to  so  reform  its  bed  and  to  so  adjust  its  slopes  as  to  restore 
normal  conditions.  In  the  meantime,  the  regulation  of  the  river 
presents  novel  and  difficult  engineering  problems,  which  will  be 
discussed  later.  It  has  been  assumed  by  many  that  human  agen- 
cies in  the  destruction  of  forests,  in  the  drainage  of  fields,  and  in 
the  building  of  levees,  are  causing  a  similar  deterioration  of  other 
rivers,  beyond  the  ordinary  area  of  deposition,  though  not  to  the 
same  extent,  and  careful  investigations  have  been  made  in  Europe 
and  the  United  States  to  determine  if  any  such  tendency  could  be 
observed. 

The  problem  is  a  difficult  one,  due  to  the  mobility  of  the  river 
bed  and  the  irregularity  of  the  rainfall.  As  the  area  of  the  cross 
section  of  the  river  is  constantly  changing,  due  to  the  movement 
of  sand  waves,  a  series  of  observations  extending  over  a  long  period 
of  time  is  necessary  to  determine  whether  an  observed  scour  or 
fill  is  due  to  a  permanent  cause,  or  is  the  accidental  result  of  the 
location  of  the  sand  wave  at  the  time  of  the  observations.  As 
the  mean  rainfall  for  even  ten-year  averages  differs  materially  at 
the  same  stations,  there  is  an  uncertainty  as  to  whether  a  river's 
height  at  any  time  has  been  due  to  a  variation  in  rainfall  or  to 
changes  in  the  river  section,  for  it  is  only  exceptional  that  sufficient 
discharge  observations  are  of  record  to  establish  any  change  of 
relation  between  gage  and  discharge. 

M.  Proney,  many  years  ago,  claimed  that  the  bed  of  the  Po 
had  risen  to  such  an  extent  through  levee  construction  as  to  render 
necessary  the  construction  of  a  new  river  channel,  to  preserve 
the  valley  from  serious  injury.  His  statement  was  questioned 
by  contemporary  Italian  engineers  (2)  and  has  been  disproved  by 


110  RIVERS  AND   HARBORS 

recent  Italian  investigators.  It  was  accepted  by  Cuvier  and  has 
been  transmitted  to  the  present  day  by  compilers  who,  while 
familiar  with  Cuvier,  are  ignorant  of  the  investigations  which  have 
resulted  from  the  statement. 

Herr  Gustav  Wex  (3),  in  a  series  of  papers  published  from  1873 
to  1879,  asserted  that  the  cutting  of  the  forests  of  Hungary  and 
Austria  had  caused  a  rise  of  the  river  beds  of  that  country,  which 
assertion  was  denied  by  Hagen  and  other  German  engineers,  who 
claimed  that  the  evidence  submitted  not  only  did  not  justify  the 
assertion,  but  that  the  little  change  in  the  river  beds  which  had 
occurred,  could  be  more  logically  accounted  for  by  the  works  of 
river  regulation  than  by  deforestation  or  levee  construction. 

Recent  investigations  have  developed  a  similar  difference  of 
opinion  among  engineers  (4),  which  when  analyzed  indicates  that 
those  who  study  large  river  systems  find  no  evidence  of  a  deterio- 
ration from  deforestation  or  from  the  drainage  of  marsh  lands, 
while  those  who  investigate  the  flow  of  mountain  streams  of  steep 
declivity  find  a  resultant  fill.  These  discrepancies  can  be  ex- 
plained by  the  fact  that  one  set  of  investigators  is  discussing  the 
portion  of  a  river  where  the  scour  and  fill  are  in  unstable  equilib- 
rium, while  the  other  group  is  considering  the  area  of  deposition. 
The  best  example  is  afforded  by  the  valley  of  the  Po,  where  a  fill  of 
the  bed  has  been  observed  in  some  of  the  tributaries  rising  in  the 
Alps,  while  the  main  stream  affords  no  evidence  of  such  action  (5) . 

As  stated  in  the  chapter  on  the  laws  of  the  flow  of  sediment, 
the  area  of  deposition  acts  as  a  large  reservoir  in  which  the  detritus 
is  retained  for  a  certain  period  and  reduced  to  a  fineness  that 
enables  it  to  be  transported  down  the  river,  without  disturbing 
the  relations  which  geological  eons  have  established.  Man's  de- 
structive work  is  ordinarily  so  puny  when  compared  with  the 
gigantic  forces  of  nature,  that  while  it  may  increase  slightly  the 
rate  at  which  the  area  of  deposition  moves  from  natural  causes 
down  a  valley,  its  effect  below  that  area  is  incapable  of  measure- 
ment. In  the  United  States  the  engineers  who  have  studied  the 
large  river  systems  have  arrived  at  the  same  conclusions  as  those 
of  the  engineers  of  Germany  and  Russia,  while  those  engaged  in 
forestry  and  those  whose  observations  are  confined  to  mountain- 
ous tributary  streams  give  illustrations  of  river  fill.  In  Germany, 
the  influence  of  forestation  even  on  small  streams  is  questioned. 
(Appendix  B.) 


FLOOD   PROTECTION  111 

It  recently  has  been  claimed  that  the  Yellow  River  (6)  of  China 
afforded  evidences  of  the  raising  of  its  bed  from  levee  construction, 
due  to  the  fact  that  when  for  ary  cause  the  river  changes  its  lo- 
cation, the  old  river  bed  is  found  at  a  higher  elevation  than  the 
one  newly  formed.  This  is  not  an  unusual  occurrence  in  any 
alluvial  river.  As  was  stated  in  explaining  the  formation  of  rivers, 
such  streams  rapidly  build  up  their  banks  from  deposits,  which 
have  a  higher  elevation  than  the  land  at  a  greater  distance  from 
the  channel.  Their  deltas  also  encroach  on  the  sea  by  deposits 
of  material  carried  down  them.  In  the  course  of  geological  ages 
they  may  so  increase  their  length  that  even  with  the  gentle  slopes 
that  usually  exist  toward  the  river's  mouth,  the  river  bed  may  be 
higher  at  the  upper  end  of  a  delta  than  the  original  land  level. 

Levee  construction,  by  retaining  in  the  river  channel  a  large 
amount  of  material  which  otherwise  would  be  deposited  on  the 
banks,  may  increase  the  deposits  in  the  sea  and  thus  accelerate 
the  lengthening  of  a  river,  but  a  change  of  slope  due  to  this  cause 
is  so  gradual  as  to  be  a  subject  of  geological  discussion  rather  than 
of  practical  river  engineering.  But  if  a  channel  is  opened  to  the 
low  areas  distant  from  the  river,  by  a  caving  of  the  high  river  banks, 
the  new  channel  will  flow  over  land  lower  than  the  river  bed, 
until  it  has  been  adjusted  to  the  new  conditions. 

Moreover,  the  diversion  of  the  great  part  of  the  discharge  from 
the  main  stream  would  cause  such  a  reduction  in  velocity  of  the 
flow  of  the  remainder,  as  to  cause  a  rapid  fill  of  the  old  bed,  and 
might  even  raise  the  crests  of  its  bars  above  the  low- water  surface, 
so  that  a  traveler  visiting  the  region  a  few  years  after  the  diversion 
occurred,  and  unfamiliar  with  the  conditions,  would  conclude  that 
there  had  been  a  greater  lowering  of  the  water  surface  than  ac- 
tually had  been  created. 

When  a  river  excavates  a  channel  across  a  narrow  neck  of  land, 
forming  what  is  called  on  western  rivers  a  cut-off,  a  similar  lower- 
ing of  the  river  bed  occurs  at  its  upper  end,  and  the  retardation  of 
the  current  around  the  bend  causes  a  large  deposit  there.  At 
the  Napoleon  cut-off  on  the  Missouri  River  which  was  made 
during  the  flood  of  1916,  such  a  fill  resulted  as  to  raise  a  great 
part  of  the  upper  end  of  the  old  channel  by  1919  above  the  mean 
river  stage. 

Such  changes  modify  the  slope  of  a  river,  and  its  discharge  will 
seek  to  regain  the  original  regimen  by  a  scour  at  some  places  and 


112  RIVERS  AND   HARBORS 

a  fill  at  others,  as  is  illustrated  by  the  Red  River,  and  explained  on 
page  96.  There  is  a  possibility,  however,  that  the  conditions  which 
exist  in  the  streams  emptying  into  Lake  Biwa,  Japan,  as  stated  on 
page  108,  may  obtain  at  Honan  on  the  Yellow  River,  since  the  latter 
also  debouches  from  a  mountainous  region  into  a  plain  at  that 
locality.  River  slopes  are  more  productive  of  changes  in  a  river 
bed  than  are  levees. 

The  necessity  for  protection  against  the  destructive  action  of 
floods  is  the  greatest,  however,  in  those  portions  of  the  river  valley 
where  the  forces  causing  deposition  and  fill  are  in  an  unstable 
equilibrium.  The  soil  created  by  the  deposit  from  alluvial  rivers  in 
such  localities  is  usually  very  fertile  and  invites  agricultural  de- 
velopment. Hence  these  localities  tend  to  become  thickly  in- 
habited, and  a  flood  not  only  destroys  the  growing  crops  but  en- 
dangers the  lives  of  the  inhabitants  and  of  their  live  stock. 

Numerous  methods  have  been  employed  or  suggested  for  pre- 
venting floods  or  for  ameliorating  their  effects.  The  first  primitive 
method  employed  was  to  build  a  mound  of  earth,  on  which  the 
settler  and  his  stock  could  take  refuge  from  the  flood  and  remain 
until  it  subsided.  When  floods  occur  at  such  seasons  of  the  year 
that  they  do  not  interfere  with  the  raising  of  crops,  this  method 
has  certain  advantages.  It  is  not  only  an  economical  method  of 
affording  relief  from  floods,  but  also  the  land  is  enriched  by  the 
annual  deposits  from  the  flood  waters,  and  the  fertility  of  the  soil 
is  maintained  without  recourse  to  fertilizers,  which  soon  become 
necessary  in  the  richest  alluvial  valley,  when  all  overflow  is  pre- 
vented. The  Nile  is  an  example  of  a  river  which  is  annually 
permitted  to  overflow  its  banks  with  most  beneficial  results,  and 
works  have  been  constructed  to  insure  an  adequate  amount  of 
water  for  this  purpose  (7). 

While  a  portion  of  the  land  on  certain  rivers  is  protected  from 
overflow  from  the  highest  floods,  another  portion  has  only  limited 
protection,  the  levees  being  given  such  a  height  as  to  permit 
overflow  above  a  certain  stage.  Certain  portions  of  the  Rhine 
have  been  thus  protected,  and  the  increased  area  over  which  the 
flood  is  allowed  to  spread  has  reduced  its  height  to  a  certain  extent. 
On  rivers  where  the  greatest  floods  are  liable  to  occur  during  the 
growth  of  vegetation,  however,  the  agricultural  interests  usually 
demand  complete  protection  at  all  stages. 

The  reduction  of  floods  by  forest  growth  has  so  many  advocates 


FLOOD   PROTECTION  113 

in  the  United  States  (8)  that  it  merits  critical  analysis.  It  is  an 
unquestioned  fact  that  a  large  amount  of  the  rain  that  falls  during 
the  period  of  the  growth  of  vegetation  is  absorbed  by  the  plants 
and  produces  their  growth.  If  that  amount  of  water  could  be 
abstracted  from  the  discharge  at  the  crest  of  a  flood,  it  would 
cause  a  perceptible  reduction  in  its  height.  It  also  is  claimed  that 
a  forest  during  its  growth  creates  a  humus  over  the  soil,  from  the 
decay  of  leaves  and  mosses,  which  will  absorb  a  large  amount  of 
rainfall;  that  the  roots  of  trees  also  loosen  the  soil  and  render  it 
more  porous  to  the  water  that  falls  on  the  surface;  that  it  retards 
the  surface  flow;  that  it  delays  the  melting  of  snow  by  shielding 
it  from  the  sun's  rays,  thus  diminishing  the  danger  of  the  produc- 
tion of  floods  by  the  snow  suddenly  adding  its  volume  of  water 
to  that  of  a  rainfall;  and  that  it  causes  a  more  uniform  distribu- 
tion of  the  rainfall,  reducing  the  intense  rains  which  produce 
floods  and  diminishing  the  period  of  drought  during  summer.  As 
an  example  Asia  Minor  is  cited,  which  has  been  converted  from 
a  fertile  region  in  ancient  tunes  to  almost  a  desert  at  the  present 
day,  the  change  being  accompanied  by  a  destruction  of  forest 
growth. 

An  analysis  of  the  causes  which  create  floods  does  not  sustain 
these  contentions.  The  moisture  which  is  absorbed  in  the  growth 
of  plants  is  derived  principally  from  the  soil  through  their  roots. 
It  therefore  is  abstracted  not  from  the  water  which  flows  on  the 
surface  but  from  that  which  has  already  percolated  into  the  ground, 
and  instead  of  reducing  the  flood  discharge  it  lessens  the  flow 
from  springs,  i.e.,  the  low-water  stage.  Moreover,  on  many  of 
the  rivers  of  the  United  States,  the  great  floods  occur  in  late 
winter  or  early  spring,  when  the  deciduous  trees  are  bereft  of 
foliage,  and  the  flow  of  sap  has  ceased  even  in  the  evergreen 
varieties. 

While  the  humus  absorbs  proportionately  more  water  than  or- 
dinary soil,  it  forms  a  reservoir  of  very  limited  depth,  and  its 
capacity  is  exceeded  by  even  moderate  rains.  It  therefore  acts 
during  the  light  rains  which  produce  low-river  stages,  but  fails 
during  the  great  storms  which  produce  floods. 

The  theory  that  the  roots  of  trees  loosen  the  soil  is  contrary 
to  fact.  A  field  whose  soil  has  been  loosened  by  plowing  absorbs 
more  rainfall  than  the  ground  under  any  variety  of  forest  growth, 
and  the  roots  of  trees  not  only  compact  the  earth  through  which 


114  RIVERS  AND   HARBORS 

they  force  their  way,  but  themselves  retard  the  flow  of  water 
which  has  been  absorbed  by  the  soil.  The  retarding  action  of 
the  forest  on  the  surface  flow  may  be  beneficial  or  injurious,  de- 
pending upon  the  superposition  of  the  flow  from  one  hillside  on 
that  from  another.  In  a  prolonged  rain  it  will  probably  have  little 
effect  in  increasing  or  diminishing  floods. 

The  influence  of  the  forest  on  snow  is  extremely  variable  (9). 
One  year  it  may  retard  its  melting  or  even  accelerate  it  so  as  to 
prevent  the  junction  of  its  water  with  that  of  a  heavy  rainfall, 
while  the  next  year  it  may  reverse  this  action  and  cause  a  super- 
position of  one  on  the  other,  depending  on  climatic  conditions 
during  the  late  winter  and  early  spring.  The  melting  of  the  snow 
by  the  sun's  rays  alone  is  so  slow  a  process  that  the  discharge  it 
creates  in  a  river  does  not  produce  floods.  If  the  sun's  rays  melt 
the  snow  before  a  heavy  rain  occurs,  the  retarding  action  of  the 
forest  is  injurious  rather  than  beneficial.  However,  it  is  by  no 
means  a  universal  law  that  the  snow  in  forests  remains  longer 
than  on  areas  bereft  of  trees.  The  snow  that  falls  in  a  forest  is 
spread  uniformly  over  the  surface  of  the  ground,  while  the  action 
of  winds  on  a  barren  hillside  tends  to  cause  the  snow  to  collect 
in  immense  drifts  which  often  remain  of  considerable  size  after 
the  snow  of  the  forest  has  disappeared.  On  the  other  hand,  a 
fall  of  sleet  which  will  create  on  the  exposed  hillside  an  impermeable 
covering  of  the  soil,  may  be  caught  by  the  leaves  and  branches  of 
trees  and  the  underlying  soil  be  thus  protected  and  rendered  more 
permeable  to  subsequent  rainfalls. 

The  effect  of  forests  on  rainfall  has  not  been  determined  accu- 
rately, but  there  is  considerable  evidence  that  there  is  a  difference 
in  precipitation  over  forests  and  over  other  areas,  as  for  example 
a  city.  But  the  great  storms  which  produce  floods  have  their 
origin  in  cyclonic  atmospheric  action  which  brings  large  amounts 
of  moisture  from  the  ocean  to  the  land.  Such  storms  have  a  path 
of  maximum  precipitation  which  is  independent  of  the  character 
of  the  vegetation  over  which  they  pass.  The  location  of  the 
mountain  ranges  has  then  a  great  influence  on  the  amount  of 
rainfall,  the  hillside  whether  barren  or  covered  with  vegetation 
receiving  more  than  the  valley. 

That  Syria  has  become  unproductive  in  recent  years  is  ascribed 
by  many  to  the  destruction  of  irrigation  works,  rather  than  to  destruc- 
tion of  forests.  But  even  admitting  for  the  sake  of  argument  the 


FLOOD   PROTECTION  115 

extreme  claims  of  the  advocates  of  reforestation,  the  reduction  of  the 
volume  of  floods  by  forest  growth  to  such  dimensions  as  would 
prevent  overflow  would  require  such  a  conversion  of  existing 
agricultural  lands  into  a  sylvan  wilderness,  that  the  country  could 
not  sustain  its  existing  population,  and  the  deer,  bear,  wolf,  and 
other  denizens  of  the  forest  would  replace  the  inhabitants  of  our 
cities  and  farms.  It  should  be  borne  in  mind  that  when  the 
forest  is  removed  other  vegetation  takes  its  place;  the  fruit  tree 
in  its  growth  absorbs  a  corresponding  amount  of  water  to  the  oak, 
and  it  is  even  possible  that  the  amount  of  water  absorbed  per  acre 
by  a  field  of  wheat  or  corn  in  producing  annually  the  stalk  and 
seed  may  exceed  that  of  a  pine  forest,  whose  growth  is  limited 
to  the  area  of  its  upper  branches. 

A  second  method  of  preventing  floods  that  has  been  proposed 
is  by  the  construction  of  large  reservoirs  (9)  to  retain  the  excess 
water  during  storms  and  to  feed  it  gradually  to  the  river  during 
low  stages.  Nature  affords  numerous  examples  of  this  method 
of  reducing  flood  heights,  of  which  the  most  noted  is  the  regula- 
tion of  the  floods  of  the  St.  Lawrence  River  by  the  Great  Lakes. 
There  are,  however,  certain  practical  difficulties  in  this  method 
of  flood  prevention  which  require  consideration.  The  most  ef- 
fective location  for  such  reservoirs  is  in  the  bed  of  the  main  river, 
but  its  alluvial  valley  is  usually  very  fertile  and  a  reservoir  of 
the  size  necessary  to  regulate  the  floods  efficiently  would  neces- 
sitate the  condemnation  of  a  rich  farming  region,  so  that  eco- 
nomical considerations  require  the  location  of  the  reservoirs  at 
the  headwaters  of  the  various  tributaries  of  the  river,  where 
the  land  is  less  adapted  to  agriculture  and  therefore  cheaper. 
This  leads  to  a  multiplicity  of  reservoirs,  and  an  increase  in  the 
volume  of  water  which  must  be  stored,  due  to  the  irregularity  of 
the  rainfall  in  the  basin.  In  one  year  the  heaviest  precipitation 
may  be  in  the  areas  drained  by  one  tributary;  in  another  year  the 
rainfall  may  be  light  in  that  basin  and  intense  in  another;  and  pro- 
vision must  be  made  for  the  storing  of  the  maximum  flow  of  each. 
A  centrally  located  reservoir  with  less  capacity  than  the  two 
combined  could  provide  storage  for  both  years. 

In  a  large  river  basin  the  limitation  that  the  reservoirs  shall  be 
located  at  the  headwaters  of  the  tributaries  on  land  not  useful 
for  farming,  leaves  a  large  area  whose  flood  waters  are  unrestricted. 
The  construction  of  an  enormous  number  of  reservoirs  in  areas 


116  RIVERS   AND   HARBORS 

where  land  is  valuable  for  agricultural  purposes  is  the  only  other 
alternative. 

The  basin  of  the  Mississippi  River  affords  an  extreme  example. 
Its  mountainous  tributaries  rise  in  the  Rocky  Mountains,  the 
Appalachian  range,  and  the  Ozarks,  from  1000  miles  to  3009 
miles  from  its  mouth;  and  if  the  entire  rainfall  of  its  mountain 
ranges  were  withdrawn  from  its  flood  discharge,  there  would  still 
remain  a  vast  region  (whose  extent  can  be  appreciated  by  a  glance 
at  a  map  of  the  United  States)  that  would  contribute  its  precipi- 
tation to  the  floods  of  the  lower  river. 

When  a  reservoir  is  built  even  in  mountainous  regions,  eco- 
nomical considerations  limit  its  capacity.  It  is  exceptional  that 
the  discharge  for  an  entire  year  or  even  during  an  entire  rainy 
season  can  be  stored  without  constructing  dams  of  inordinate 
height.  Ordinarily  the  reservoir  must  be  emptied  after  every 
great  storm  to  supply  space  for  the  flow  of  one  which  succeeds  it. 

This  necessity  is  a  serious  objection  to  the  combined  employ- 
ment of  reservoirs  to  reduce  flood  heights  and  also  to  store  water 
for  the  production  of  power.  The  two  purposes  are  antagonistic. 
The  production  of  water  power  demands  a  conservation  of  the 
water  until  low  stages  so  as  to  preserve  as  constant  a  head  as 
practicable.  When  once  filled  the  reservoir  should  remain  so 
until  low  water  and  the  surplus  from  subsequent  precipitations 
should  be  allowed  to  escape,  which  would  interfere  seriously  with 
its  utilization  for  flood  protection.  If  it  is  emptied,  the  antici- 
pated storm  may  not  materialize  and  the  reservoir  may  be  useless 
for  power  purposes  for  the  remainder  of  the  season. 

Where  a  system  of  reservoirs  only  partially  controls  the  dis- 
charge of  a  basin,  due  to  the  irregularity  of  the  rainfall,  there  is 
danger  of  the  discharge  from  the  reservoir  arriving  in  the  lower 
valleys  of  a  river,  when  the  unregulated  floods  from  the  other 
tributaries  are  at  their  highest  stage,  thus  increasing  their  height. 
If  the  flood  in  the  mountains  had  been  uncontrolled  by  reservoirs, 
its  flood  waters  might  have  passed  down  the  main  stream  prior  to 
the  arrival  of  the  floods  of  the  tributaries  from  prairie  regions. 

Employing  the  Mississippi  River  basin  again  as  an  extreme  ex- 
ample, a  series  of  reservoirs  can  be  constructed  at  the  headwaters 
of  the  Missouri  River,  which  would  retain  all  the  water  derived 
from  a  heavy  precipitation  and  could  deliver  it  after  all  danger 
from  that  rainfall  had  subsided  within  a  bank-full  stage.  But  it 


FLOOD   PROTECTION  117 

would  require  about  40  days  for  the  water  after  it  had  been  re- 
leased to  flow  to  the  Mississippi.  If  heavy  rains  should  occur  in 
Nebraska,  Kansas,  Iowa,  and  Missouri  which  would  cause  the 
lower  tributaries  of  the  Missouri  to  have  a  maximum  discharge 
when  the  reservoirs  were  producing  a  bank-full  stage  in  the  main 
river,  a  great  flood  would  result  which  would  have  been  avoided 
if  the  original  flood  from  the  mountains  had  been  permitted  to 
escape  without  restraint,  and  if  the  main  river  was  again  at  a  low 
stage  when  the  lower  tributaries  were  in  flood. 

Such  a  combination  on  the  Missouri,  however,  would  be  excep- 
tional, as  the  floods  of  the  lower  tributaries  usually  precede  those 
of  the  mountain  streams,  and  the  discharge  of  the  latter  is  small 
compared  to  that  of  the  former.  It  would  be  more  liable  to  occur 
in  the  Ohio  Valley,  although  the  flood-wave  moves  from  Pitts- 
burgh to  Cairo  in  about  ten  days,  and  a  delayed  flood  from  the  Ohio 
whose  crest  corresponded  at  Cairo  with  that  of  the  Missouri  River, 
which  usually  arrives  at  that  point  at  a  later  date,  would  produce 
most  disastrous  results  in  the  lower  Mississippi  Valley,  since  the 
combined  maximum  discharges  of  the  two  rivers  far  exceed  any 
discharge  recorded  on  the  lower  river. 

Nor  is  it  a  proper  reply  to  the  criticism  that  such  a  combination 
would  be  exceptional.  It  cannot  be  too  strongly  emphasized  that 
average  conditions  produce  average  stages  and  that  great  floods 
always  arise  from  exceptional  conditions. 

When  the  entire  basin  is  regulated  by  reservoirs,  the  engineer 
in  charge  of  the  system  has  a  most  complicated  problem  to  solve. 
(In  a  project  for  regulating  the  Kaw  Valley,  Kansas,  over  70  reser- 
voirs are  proposed.)  After  every  storm  he  must  reduce  the  level 
of  all  the  reservoirs  to  a  predetermined  elevation  before  the  arrival 
of  the  next  rainfall,  and  he  must  not  permit  the  discharge  from 
each  reservoir  to  overflow  the  banks  of  the  tributary  on  which 
it  is  situated,  and  the  combined  flow  of  the  various  tributaries 
must  not  exceed  that  of  the  bank-full  stage  of  the  main  river. 
As  the  time  of  the  arrival  of  the  next  storm  and  its  intensity  are 
unknown,  he  does  not  possess  the  requisite  data  on  which  to  base 
his  computations.  In  the  United  States  the  retarding  basin  has 
therefore  been  substituted  recently  for  the  retention  reservoir. 

The  retarding  basin  (10)  is  formed  by  the  construction  of  a 
barrier  across  a  valley  which  does  not  interfere  with  the  low- 
water  discharge  of  the  stream,  but  limits  the  flood  discharge  to  a 


118  RIVERS  AND   HARBORS 

predetermined  amount.  One  of  the  first  examples  of  this  type  of 
construction  is  afforded  by  the  Pinay  dam  across  the  valley  of 
the  Loire  River  in  France.  When  a  flood  occurs,  the  portion  of 
the  valley  selected  for  the  retarding  basin  is  overflowed  to  a  higher 
stage  than  otherwise  would  exist.  Since  the  flow  through  the 
barrier  is  reduced,  the  flood  heights  in  the  lower  portions  of  the 
valley  are  diminished,  but  as  the  river  is  permitted  to  return  to 
its  normal  condition  during  low  stages,  the  lands  in  the  upper 
reaches  of  the  river  are  overflowed  only  temporarily  during  high 
stages,  and  they  still  can  be  utilized  for  such  agricultural  purposes 
as  raising  a  crop  of  hay  or  for  pasturage.  The  cost  of  land  condem- 
nation is  therefore  less  than  when  a  reservoir  of  corresponding 
dimensions  is  constructed,  since  only  a  temporary  use  of  the  lands 
affected  by  the  increased  heights  of  the  restrained  flood  over  or- 
dinary overflow  has  to  be  obtained,  instead  of  a  purchase  of  the 
entire  area  flooded. 

It  also  has  been  attempted  to  reduce  flood  heights  by  enlarging 
the  low-water  section  of  the  river.  The  success  of  this  method 
is  dependent  largely  upon  the  amount  of  sediment  the  river  carries 
in  suspension.  In  alluvial  rivers  the  low-water  bed  is  the  result 
of  a  conflict  between  the  forces  that  cause  deposition  and  those 
that  cause  scour.  If  the  equilibrium  which  these  forces  have 
created  is  destroyed  by  artificial  means,  there  is  a  tendency  to 
return  to  normal  conditions,  and  periodic  dredging  is  required  to 
maintain  the  enlargement.  In  a  valley  formed  of  glacial  drift, 
however,  an  enlarged  section  may  afford  considerable  relief. 

Another  method  proposed  for  reducing  flood  heights  is  by 
straightening  the  river  (11).  This  method  of  flood  reduction  is 
successful  only  when  the  excavated  channel  extends  to  a  tidal 
bay  or  a  lake.  If  it  connects  one  portion  of  a  river  with  another 
it  causes  merely  a  local  lowering  of  the  water  surface  at  the  upper 
end  of  the  cut,  and  a  corresponding  raising  of  flood  heights  at  the 
lower  end,  similar  to  that  which  is  caused  in  the  low- water  channel 
by  analogous  means.  There  is  also  the  same  tendency  to  excessive 
caving  in  alluvial  soils.  In  fact  the  deterioration  of  the  low-water 
channel  usually  occurs  during  floods,  including  the  movement  of 
a  large  amount  of  sediment  into  the  channel  below  the  cut,  the 
reduction  of  slope  through  it,  and  the  increase  of  slope  above. 
Humphreys  and  Abbot,  in  their  Physics  and  Hydraulics  of  the 
Mississippi  River,  estimate  the  increased  height  of  the  flood  below 


FLOOD    PROTECTION  119 

the  straightened  channel  to  be  equal  to  half  the  fall  in  a  straight 
portion  equal  in  length  to  the  shortening  of  the  channel. 

The  reduction  of  flood  heights  by  means  of  outlets  (12)  and  waste 
weirs  also  has  been  proposed.  Since  an  outlet  reduces  the  dis- 
charge of  the  river  at  all  stages,  it  is  injurious  to  the  regimen  of 
the  river  at  low  water  when  as  great  a  discharge  as  practicable 
is  required  to  maintain  the  low-water  channel.  The  water  flow- 
ing through  the  outlet  has  also  to  be  prevented  from  overflowing 
the  country,  and  usually  the  cost  of  levee  construction  along  its 
banks  will  exceed  the  extra  cost  of  making  the  levees  along  the 
main  stream  of  sufficient  height  to  carry  the  entire  river  dis- 
charge. A  system  of  lakes  through  which  the  outlet  flows  may 
reduce  flood  heights  in  the  outlet,  however,  to  such  an  extent 
that  the  combined  levee  system  is  more  economical  than  a  single 
one. 

There  is  a  fundamental  principle,  moreover,  which  condemns  the 
use  of  outlets  on  alluvial  rivers,  with  but  few  exceptions.  When 
a  river's  flow  is  divided  between  two  channels,  there  is  a  difference 
of  velocity  in  the  two  branches,  and  there  is  a  tendency  to  the 
deposition  of  sediment  in  the  one  that  has  the  least  velocity. 
The  latter  channel  thereby  contracts  and  the  other  channel  tends 
to  enlarge.  An  outlet  therefore  will  fill  or  scour,  according  as  its 
velocity  is  less  than  or  greater  than  that  of  the  main  stream.  If 
it  tends  to  fill,  it  can  be  kept  open  only  by  dredging.  If  it  scours, 
there  is  danger  that  it  will  become  the  main  channel,  and  that 
the  old  river  bed  will  be  abandoned.  In  the  lower  Mississippi 
Valley,  the  outlets  have  all  shown  a  tendency  to  fill  and  close 
themselves,  though  in  some  instances  this  tendency  has  been  ac- 
celerated by  the  construction  of  levees  across  their  heads.  Even 
the  Atchafalaya  outlet,  which  is  required  for  the  navigation  of  the 
Red  River,  can  be  maintained  at  low  water  only  by  dredging.  As 
a  precautionary  measure,  however,  submerged  dams  of  brush  and 
stone  have  been  constructed  across  its  head  to  prevent  undue 
enlargement  during  floods. 

A  waste  weir  or  spillway  is  a  structure  in  a  levee  line  which 
permits  the  discharge  of  water  above  a  bank-full  stage  and  does 
not  produce  as  injurious  effects  on  the  low- water  channel  as  an 
outlet.  The  water  which  escapes  through  a  waste  weir  has  also 
to  be  prevented  from  overflowing  the  adjoining  land,  and  its 
channel  has  to  be  limited  by  levees.  There  is  also  the  same 


120  RIVERS  AND   HARBORS 

tendency  to  a  deposit  of  sediment  as  in  an  outlet,  and  periodic 
dredging  is  required  to  maintain  its  efficiency. 

In  the  Sacramento  Valley  (13)  waste  weirs  have  been  employed 
extensively  to  reduce  flood  heights,  which  had  become  excessive 
on  account  of  the  passage  down  the  valley  of  sand  waves  caused 
by  hydraulic  mining.  The  river  is  in  a  state  of  transition  from 
the  old  equilibrium  which  existed  prior  to  the  introduction  of 
hydraulic  mining  to  a  new  one  which  it  is  now  attempting  to 
create,  and  its  local  slopes  are  constantly  changing.  These  waste 
weirs  lead  to  broad-passes  on  the  opposite  side  of  the  valley  to 
that  in  which  the  river  channel  is  located.  These  by-passes  un- 
questionably will  afford  places  of  deposit  for  large  masses  of  the 
detritus  moving  down  the  river,  but  it  is  considered  probable  that 
sufficient  sand  will  continue  to  move  in  the  river  bed  so  that  ex- 
tensive dredging  to  maintain  the  low-water  channel  will  be  re- 
quired for  many  years  until  the  new  equilibrium  is  established. 

If  dredging  were  suspended  at  the  juncture  of  the  Mississippi, 
Red,  and  Atchafalaya  rivers,  natural  causes  would  soon  convert 
this  outlet  of  the  Mississippi  River  into  a  waste  weir.  To  maintain 
navigation  between  the  Mississippi  and  the  Red  rivers  (14)  would 
then  require  the  construction  of  an  expensive  lock  and  a  short 
canal.  The  flow  through  the  dredged  connecting  channel  is 
small  when  compared  with  the  low-water  discharge  of  the  Missis- 
sippi, and  the  shoaling  of  the  main  river  resulting  from  the  small 
low-water  diversion  through  the  outlet  is  slight.  The  advisability 
of  converting  the  outlet  into  a  waste  weir  therefore  becomes  a 
question  of  the  relative  cost  of  maintaining  a  dredged  channel, 
and  of  the  interest  on  the  construction  and  maintenance  of  a  lock 
and  canal. 

The  method  of  flood  protection  that  is  employed  most  uni- 
versally is  the  construction  of  levees.  This  method  has  stood  the 
test  of  practical  application  for  ages.  The  principal  objection 
to  its  use  is  the  increase  in  flood  heights  that  is  caused  by  the  con- 
finement of  the  overflow  to  the  river  channel.  Many  engineers 
claim  that  the  concentration  of  the  flood  discharge  in  the  river 
bed  will  produce  its  enlargement  so  as  ultimately  to  remove  this 
objection,  but  observations  over  long  periods  on  leveed  rivers 
have  demonstrated  that  the  increase  in  the  cross-sections  of  the 
river  channel  is  so  gradual  from  this  cause  as  not  to  merit  practical 
consideration.  The  levees  must  be  built  to  the  maximum  height 


FLOOD   PROTECTION  121 

to  produce  the  scouring  effect  desired  or  they  will  be  overflowed 
and  destroyed  before  it  is  effected.  Any  subsequent  enlargement 
of  river  section  merely  adds  a  factor  of  safety  to  a  levee  line  already 
constructed. 

While  a  levee  performs  the  same  function  as  a  reservoir  embank- 
ment, it  differs  from  an  ordinary  earthen  dam  both  in  form  and  in 
method  of  construction.  For  economical  reasons  it  must  be  built 
from  the  soil  in  the  vicinity,  and  there  is  rarely  available  material 
to  form  the  puddled  core  so  essential  in  reservoir  construction. 
Moreover,  it  usually  rests  on  a  permeable  foundation  through 
which  water  would  percolate  even  if  the  embankment  were  made 
impermeable.  The  necessary  seepage  must  be  reduced  to  such 
an  extent  that  the  embankment  is  not  endangered  by  planes  of 
saturation  through  its  having  such  slopes  that  the  superincumbent 
dry  material  will  tend  to  slide.  Moreover,  the  water  passing 
through  or  under  the  levee  must  not  be  permitted  to  flow  with 
sufficient  force  to  move  particles  of  the  material  of  which  the  levee 
is  composed  or  that  on  which  it  rests.  This  requires  gentler 
slopes  on  the  land  side  than  usually  are  formed  in  reservoir  dams. 
The  exact  dimensions  depend  on  the  character  of  the  alluvium 
of  the  valley  to  be  protected. 

On  the  lower  Mississippi  (15),  levees  usually  are  given  a  height 
of  three  feet  above  the  estimated  highest  flood,  a  width  of  crown 
of  eight  feet  and  a  land  slope  of  one  on  three,  for  levees  less  than 
12  feet  in  height.  For  levees  of  a  greater  height,  it  is  necessary 
to  increase  the  width  of  the  base,  generally  by  the  addition  of  a 
banquette,  which  extends  from  a  point  on  the  land  slope  with 
an  elevation  eight  feet  below  that  of  the  crown.  Its  width  varies 
from  20  to  40  feet,  depending  on  the  height  of  the  levee.  Its  upper 
slope  is  about  1  on  10  to  provide  for  a  proper  drainage  of  rain 
water,  and  its  land  slope  is  1  to  4. 

This  arrangement  provides  a  width  of  base  exceeding  10  times 
the  head  under  which  the  water  flows  through  the  subsoil.  This 
is  somewhat  less  than  is  allowable  in  a  dam  founded  on  permeable 
soil  in  a  river  bed.  Since  the  river  is  at  an  extreme  flood  stage  for 
a  comparatively  short  time,  however,  so  large  a  factor  of  safety 
is  not  requisite.  In  exceptional  cases  sand  boils  have  developed 
behind  the  levee  line  during  floods,  rendering  necessary  a  further 
extension  of  the  banquette. 

The  river  slope  is  usually  one  to  three,  which  is  sufficiently 


122  KIVERS  AND   HARBORS 

gentle  to  afford  protection  from  erosion  from  rains  and  river  cur- 
rents when  the  slope  is  well  sodded  with  Bermuda  grass.  How- 
ever, if  the  levee  is  exposed  to  wave  action  during  storms  or  from 
passing  vessels,  either  a  gentler  slope  or  a  more  effective  protec- 
tion must  be  provided.  In  such  locations  a  facing  of  concrete  is 
frequently  employed.  The  crown  and  land  slopes  of  the  levee 
are  protected  by  Bermuda  sod. 

In  European  countries,  the  levees  have  approximately  the  same 
width  of  base  as  those  on  the  Mississippi  River,  but  they  have  a 
much  wider  crown  and  much  steeper  slopes.  This  is  on  account 
of  the  common  use  of  the  levees  as  roadways,  for  which  purpose 
the  crowns  of  the  levees  are  paved.  The  American  form  of  levee 
secures  protection  from  floods  with  less  earth  than  those  of  Eu- 
rope, and  when  road  construction  has  advanced  to  the  stage  of 
substituting  some  form  of  pavement  for  the  dirt  road,  the  banquette 
will  afford  a  proper  place  for  its  location  with  far  less  expenditure 
than  if  it  were  placed  on  the  top  of  the  levee. 

An  important  item  of  levee  construction  which  is  too  often 
neglected  is  a  drainage  ditch  on  the  land  side  of  the  levee  to  pre- 
vent the  seepage  water  from  injuriously  affecting  the  crops  in  the 
adjoining  fields. 

The  methods  employed  in  levee  construction  vary  from  the 
laborer  with  his  spade  and  wheelbarrow  to  elaborate  steam  levee 
machines  (16)  and  the  hydraulic  dredge.  The  principal  economic 
factor  in  the  problem  is  the  height  of  the  levee.  In  a  small  levee 
of  short  length,  manual  labor  may  be  the  cheaper,  due  to  overhead 
charges  for  plant;  as  the  levee  increases  in  size,  animal  traction  is 
profitably  substituted ;  for  the  large  levees  on  the  lower  Mississippi 
the  levee  machines  become  the  most  economical  means  of  con- 
struction. 

The  hydraulic  dredge  which  derives  the  material  for  the  levee 
from  the  river  bed  is  economical  when  the  lift  from  the  low-water 
surface  to  the  crown  of  the  levee  is  relatively  small  and  the  levee 
is  located  close  to  the  river  bank.  It  has  been  employed  exten- 
sively for  levee  construction  on  the  Sacramento  and  on  the  upper 
Mississippi. 

The  flood  protection  of  the  Miami  River,  Ohio,  (17),  is  an 
example  of  a  judicious  application  of  the  principles  enumerated 
above.  It  is  estimated  that  the  maximum  flood  discharge  of  the 
Miami  River  if  uncontrolled  may  equal  350,000  second-feet  at 


FLOOD   PROTECTION  123 

Dayton  and  490,000  second-feet  at  Hamilton.  By  retarding 
basins  the  crest  of  the  flood  is  reduced  to  125,000  second-feet  at 
Dayton  and  200,000  second-feet  at  Hamilton,  a  discharge  about 
one-third  greater  than  the  natural  channel  capacity  of  the  river 
at  either  locality.  This  is  provided  for  by  an  enlargement  of  the 
low-water  bed  and  by  levee  construction. 

The  estimated  cost  of  the  work  was  $25,000,000,  while  the  cost 
of  detention  basins  which  would  have  reduced  flood  heights  to 
the  existing  channel  capacity  of  the  river  was  estimated  at 
$96,000,000. 

The  Miami  River  carries  relatively  little  sediment  in  suspension, 
and  a  large  amount  of  the  material  that  moves  along  the  river 
bed  during  floods  will  be  retained  in  the  detention  basins.  Hence 
the  danger  of  refilling  the  enlarged  channel  is  not  so  great  as  it  is 
in  an  alluvial  river  heavily  charged  with  sediment. 

The  preceding  analysis  leads  to  the  following  conclusions  re- 
garding the  proper  method  of  treatment  of  the  flood  waters  of  a 
large  river  basin. 

On  steep  mountain  slopes,  the  destruction  of  forest  growth 
should  be  prevented  in  order  to  preserve  the  soil  from  excessive 
erosion,  just  as  the  steep  slopes  of  a  levee  are  protected  from  rain 
wash.  It  is  not  necessary  to  prevent  the  cutting  of  trees  to 
attain  this  object,  as  the  stumps  and  roots  are  the  protective 
agencies.  Until  they  decay  they  will  prevent  scour  as  effectively 
as  the  live  tree.  The  cut  over  land,  however,  must  be  protected 
from  forest  fires,  the  removal  of  the  stumps  and  roots  prohibited, 
and  a  second  growth  of  timber  encouraged. 

The  mountainous  valleys  within  the  area  of  deposition  usually 
contain  relatively  little  land  suitable  for  agriculture,  and  are  fre- 
quently of  such  shape  that  dams  can  be  constructed  readily  which 
will  create  reservoirs  of  large  capacity.  Under  such  conditions 
power  development  should  become  the  controlling  factor.  Private 
capital  is  prepared  to  construct  the  dams  if  granted  a  franchise. 
The  general  public,  however,  is  entitled  to  some  return  for  per- 
mitting such  construction,  and  the  power  company  equitably  can 
be  required  to  build  dams  of  such  height  that  larger  volumes  of 
water  can  be  stored  than  are  required  for  power  purposes.  The 
excess  water  can  be  employed  to  regulate  stream  flow.  As  the 
valleys  increase  in  width  and  in  fertility,  the  retarding  basin  affords 
an  economical  method  of  reducing  flood  heights.  It  converts  the 


124  RIVERS  AND   HARBORS 

flood  wave  as  shown  on  the  river's  hydrograph  from  a  series  of 
sharp  peaks  with  intervening  depressions  to  a  smoother  curve, 
diminishing  the  height  of  the  flood  but  increasing  its  duration. 
When,  however,  by  the  superposition  of  the  flow  of  many  tribu- 
taries, the  river's  hydrograph  has  been  flattened  by  natural  flow, 
a  further  lowering  of  the  flood  crest  requires  the  storage  of  such 
a  volume  of  water  as  to  require  the  occupancy  of  too  much  agri- 
cultural land  by  the  storage  basin  for  the  economical  protection 
of  the  remainder  of  the  valley.  In  that  case,  a  levee  system  be- 
comes the  cheapest  method  of  flood  protection.  It  may  be  that 
the  reduced  floods  from  numerous  tributaries,  controlled  by  re- 
tarding basins,  will  occasionally  so  combine  as  to  increase  the  re- 
sultant flood  on  the  lower  river.  If  so,  it  is  a  penalty  the  lower 
valley  incurs  from  its  location,  and  the  increased  size  of  its  levees 
becomes  necessary  as  the  result  of  insuring  protection  to  the  ]ow- 
lands  of  the  tributaries. 


CHAPTER  XII 

ESTUARIES 

When  a  river  empties  into  the  ocean,  it  is  exposed  to  the  influence 
of  the  tides,  which  frequently  exert  a  greater  force  than  the  dis- 
charge. In  mid-ocean  the  tidal  wave  causes  a  relatively  small  rise 
and  fall  of  the  water  surface,  but  as  the  wave  approaches  a  coast 
the  oscillations  increase  in  size  and  extend  up  estuaries  for  long 
distances.  At  the  mouths  of  rivers  the  slope  due  to  the  river  flow 
is  gentle,  frequently  less  than  0.1  foot  per  mile.  The  river  en- 
counters a  fluctuation  in  sea-level  which  varies  from  14  inches  in 
the  Gulf  of  Mexico  to  28  feet  in  some  localities  around  the  British 
Islands,  and  in  extreme  cases,  as  in  the  Bay  of  Fundy,  the  fluctua- 
tion may  exceed  50  feet.  At  low  tides  the  natural  flow  of  the  river 
is  greatly  accelerated,  while  at  high  tides,  not  only  is  the  outflow 
of  the  river  water  prevented,  but  there  is  often  a  large  inflow  of 
water  from  the  ocean.  Since  salt  water  has  a  greater  density  than 
fresh  water,  the  inflowing  tide  of  salt  water  at  certain  stages  moves 
along  the  river  bed  with  an  outflow  of  river  water  above  it. 

In  the  St.  Lawrence  River  (1)  the  tidal  wave  is  propagated  up 
the  river  a  distance  of  350  miles  at  the  rate  of  83  miles  per  hour. 
Even  the  relatively  small  tides  of  the  Gulf  of  Mexico  produce 
during  low  water  an  appreciable  effect  on  the  flow  of  the  Mississippi 
River  at  Red  River  Landing,  300  miles  from  its  mouth. 

The  height  of  the  tidal  wave  in  a  river  is  a  function  not  only  of 
the  height  of  the  ocean  tide,  but  also  of  the  form  of  the  estuary. 
If  a  river  empties  into  a  sea  through  a  wide  funnel-shaped  mouth, 
the  height  of  its  tides  is  greatly  increased.  In  the  Gulf  of  St. 
Lawrence  the  tidal  range  is  from  3  to  4  feet,  while  at  Quebec  it 
varies  from  9  to  18  feet.  In  the  Thames  the  level  of  high  water  at 
London  Bridge  is  nearly  4  feet  higher  than  at  its  mouth  (2).  If 
the  river  outlet  is  contracted  from  any  cause,  the  height  of  the 
tidal  wave  is  diminished.  Thus  the  contracted  entrance  to 
Chesapeake  Bay  reduces  the  height  of  the  tides  of  the  rivers 
emptying  into  it,  but  the  funnel-shaped  mouths  of  the  Potomac 

125 


126  RIVERS  AND   HARBORS 

River  and  the  James  River  cause  higher  tides  at  the  head  of  navi- 
gation of  these  rivers  than  exist  in  the  bay. 

The  rate  of  propagation  of  the  tidal  wave  is  a  function  of  the 
depth.  In  mid-ocean  it  has  a  motion  of  about  600  miles  per  hour. 
As  it  approaches  a  coast  not  only  is  the  oscillation  increased  but  the 
rate  of  propagation  is  diminished.  The  speed  does  not  exceed 
100  miles  per  hour  in  depths  of  100  feet.  In  rivers  it  is  still  further 
reduced  by  shoals.  At  the  mouth  of  the  Seine,  when  the  depth  was 
5.9  feet,  the  rate  of  propagation  up  the  river  was  9.8  miles  per  hour. 
Increasing  the  depth  of  17.7  feet  increased  the  rate  of  propagation 
to  16.4  miles  per  hour  (3). 

The  tidal  wave  is  caused  primarily  by  water  pressure.  It  is 
transmitted  in  the  same  manner  that  the  flood  wave  is  propagated 
in  non-tidal  rivers.  It  is  not  dependent  on  the  velocity  of  the 
river  flow,  which  is  a  function  of  the  slope  of  its  surface  and  the 
energy  imparted  to  the  water.  Even  where  tidal  oscillations  are 
large,  the  velocity  of  the  tidal  currents  rarely  exceeds  five  miles 
per  hour.  Excessive  velocities  result  from  obstructions  to  the 
transmission  of  the  tidal  oscillation.  Where  a  shallow  bar  is  formed 
at  the  mouth  of  a  river,  the  tide  rises  much  more  rapidly  in  the 
sea  than  in  the  river,  producing  a  steep  slope  over  the  bar.  At 
certain  periods  of  the  tide,  the  tidal  oscillation  may  break  over  the 
bar  as  a  wave  breaks  on  the  seashore  instead  of  moving  up  the 
river  as  a  wave.  This  produces  what  is  termed  a  tidal  bore  and 
causes  a  supplemental  surface  wave  to  flow  up  the  river  with  great 
velocity.  Among  the  most  noted  bores  is  that  found  at  the  mouth 
of  the  Tsien-Tang-Kiang  River  (4)  in  China.  At  certain  tidal  stages 
this  bore  creates  a  slope  of  1  foot  to  the  mile  for  a  distance  of  20 
miles  and  a  surface  velocity  of  20  feet  per  second.  In  the  Petit 
Codiac  River  emptying  into  the  Bay  of  Fundy,  a  bore  wave  5  feet 
4  inches  high,  moving  at  the  rate  of  8.47  miles  per  hour,  has  been 
observed.  Bores  are  created  at  the  mouths  of  numerous  other 
rivers,  particularly  during  spring  tides,  and  it  has  also  been  ob- 
served that  if  a  deep  channel  is  formed  across  the  obstructive  bar, 
the  bore  may  be  eliminated. 

As  the  result  of  the  tidal  oscillation  the  flow  of  rivers  within  tidal 
influence  is  constantly  changing.  As  the  tide  rises,  the  outflow  is 
checked  and  is  succeeded  by  a  period  during  which  the  water 
ceases  to  flow.  This  is  followed  by  an  inflow  from  the  sea  which  is 
reduced  to  zero  at  high  tide,  and  is  succeeded  by  an  outflow  as  the 


ESTUARIES  127 

tide  falls.     During  both  flood  and  ebb  tides  the  velocity  of  the 
flow  is  greatest  at  half  tide. 

As  the  flood  tide  enters  a  river  it  encounters  in  the  low-water 
tidal  basin  the  fresh  water,  which  is  increased  by  the  river's 
discharge.  The  tide  has  to  force  this  water  upstream  in  advance 
of  its  flow.  Hence  the  inflow  from  the  sea  would  ascend  a  river  a 
relatively  short  distance  if  it  were  not  for  the  difference  in  density 
of  salt  and  fresh  water,  which  causes  the  river  water  to  rise  to  the 
surface  and  the  ocean  water  to  flow  along  the  river  bed  until  they 
have  become  mixed.  While  the  inflow  of  sea  water  is  limited,  the 
motion  it  imparts  to  the  water  that  it  backs  up  extends  to  a  point 
where  the  river's  discharge  cannot  be  overcome.  The  discharge 
of  the  Mississippi  River  is  so  great  during  floods  that  not  only  is  the 
tidal  flow  unable  to  enter  the  river,  but  its  discharge  displaces  the 
waters  of  the  Gulf  of  Mexico  for  a  considerable  distance  from  its 
mouth.  Since  the  flood  tide  impounds  the  river  discharge  while 
the  ebb  tide  flows  with  it,  more  water  is  discharged  during  the  ebb 
than  enters  during  the  flood.  The  durations  of  the  flood  and  ebb 
tides  also  differ.  At  the  mouths  of  large  estuaries  the  duration  of 
the  flood  tide  is  about  5J  hours  and  of  the  ebb  tide  about  6 \  hours. 
The  difference  in  the  duration  increases  with  the  distance  from 
the  mouth,  and  with  the  frictional  resistance  to  the  progress  of  the 
tidal  wave  due  to  the  shoaling  of  the  water  and  to  the  narrowing 
of  the  channel  (5). 

As  a  result  of  this  tidal  action,  a  river's  discharge  moves  inter- 
mittently through  its  tidal  estuary,  with  its  velocity  increased 
during  the  ebb  tides  and  reduced  during  the  flood  tides.  When 
the  flow  is  checked,  there  is  a  tendency  to  deposit  the  sediment  it 
carries,  which  is  again  moved  along  the  river  bed  when  the  current 
is  accelerated.  Moreover,  the  inflow  from  the  sea  brings  silty 
material  into  the  river  from  the  sand  waves  which  are  moving 
along  the  coast,  and  this  silt  has  also  an  intermittent  motion  up 
and  down  the  tidal  basin. 

There  is  a  marked  difference  in  the  method  by  which  the  silt  is 
transported  in  the  non-tidal  sections  of  a  river  and  in  its  estuary. 
In  the  upper  portions  of  a  river  the  great  mass  of  the  material 
either  is  carried  in  permanent  suspension,  or  moves  as  sand  waves  in 
contact  with  its  bed  associated  with  a  small  amount  temporarily  in 
suspension.  In  a  tidal  basin,  on  account  of  the  greater  specific 
gravity  of  sea  water  and  its  tendency  to  flow  along  the  river  bed, 


128  RIVERS  AND  HARBORS 

the  material  eroded  is  mixed  more  intimately  with  the  water.  A 
much  larger  percentage  is  placed  intermittently  in  suspension  and 
a  comparatively  small  amount  moves  as  sand  waves.  The  material 
which  has  been  carried  in  permanent  suspension  in  the  upper  por- 
tions of  the  river  is  deposited  during  slack  water  and  thereafter 
also  moves  intermittently  in  suspension.  This  silt  moves  back 
and  forth  with  the  tides  along  the  river  bed,  and  where  for  any 
cause  there  is  an  obstruction  to  the  free  propagation  of  the  tidal 
wave,  it  tends  to  accumulate  and  form  a  shoal. 

The  curved  trac6  with  its  pools  in  bends  and  its  crossings  over 
bars  between  the  pools,  which  is  so  essential  to  the  regulation  of  the 
non-tidal  portions  of  a  river,  as  explained  in  Chapter  VI,  is  inapplica- 
ble to  the  improvement  of  its  estuary.  In  the  upper  reaches  of  a 
river  the  formation  of  a  bar  in  the  crossing,  with  the  resultant  slope 
over  it,  is  necessary  to  preserve  the  depths  in  the  pools,  and  to 
prevent  an  injurious  increase  of  slope  on  the  bars  above  and  below 
it.  In  the  tidal  section,  however,  the  formation  of  a  permanent 
shoal  not  only  limits  navigation  over  it  but  reduces  the  tidal  flow 
and  therefore  affects  channel  depths  in  other  localities.  With  the 
constant  fluctuation  of  the  tides,  there  can  be  no  permanency  of 
river  slope  in  any  portion  of  the  estuary.  A  bar  steepens  the  slope 
locally  during  certain  portions  of  the  tide,  with  a  corresponding 
reduction  of  slope  at  other  localities.  A  bend  not  only  tends  to 
form  a  bar  below  it,  but  it  also  offers  a  resistance  to  the  flow  of 
water  by  forcing  it  to  change  its  direction,  thus  interfering  with  the 
tidal  flow.  In  a  curved  channel  there  is  also  a  tendency  for  the 
flood  tide  and  the  ebb  tide  to  follow  different  paths,  causing  a  long 
period  of  slack  water  in  each  path,  during  which  silt  is  depos- 
ited. This  action  produces  cross-currents  injurious  to  the  main 
channel. 

In  the  improvement  of  an  estuary,  the  channel  should  therefore 
be  made  as  straight  as  possible,  and  any  bends  that  it  is  necessary 
to  introduce  should  be  of  gentle  curvature.  When  the  tidal  inflow 
largely  exceeds  the  river's  discharge,  it  becomes  the  controlling 
factor  in  determining  channel  depths  and  should  be  restricted  as 
little  as  practicable.  This  necessitates  a  gradual  enlargement  of 
the  channel  of  the  estuary  from  the  head  of  the  tidal  flow  to  the 
river's  mouth,  proportioned  to  the  tidal  discharge. 

Where  it  has  been  attempted  to  reverse  the  process  and  prevent 
the  entrance  of  the  flood  tide,  as  at  the  port  of  Boston,  England, 


ESTUARIES  129 

where  the  river  Witham  was  closed  by  a  sluice-gate,  a  disastrous 
shoaling  has  occurred  (6).  In  such  cases  the  incoming  tide  has  its 
velocity  checked  by  the  barrier  and  a  long  period  of  slack  water  is 
created.  During  this  period  any  material  held  in  suspension  is 
deposited,  and  the  ebb  flow  has  not  sufficient  force  to  remove  it. 
Moreover,  the  effect  of  the  barrier  extends  for  long  distances  below 
it  and  may  even  have  an  injurious  action  on  the  bar  at  the  river's 
mouth.  There  results  as  a  corollary  another  principle  in  improving 
estuaries:  The  tidal  flow  should  be  admitted  as  far  up  a  river  as 
possible  and  all  barriers  to  its  progress  removed  so  that  the  period  of 
slack  water  may  be  reduced  to  a  minimum  (6). 

In  Chapter  II  attention  was  called  to  the  influence  of  the  geo- 
logical formation  of  non-tidal  rivers  on  the  flow  of  sediment.  This 
is  still  more  evident  in  their  tidal  estuaries.  A  river  that  empties 
into  the  ocean  between  two  rocky  promontories  or  into  a  bay 
similarly  protected,  is  exposed  to  the  movement  of  comparatively 
little  material  along  the  ocean  bed,  and  will  have  a  deep  mouth. 
If  the  river  empties  into  the  ocean  along  a  sandy  coast,  however, 
a  bar  is  formed  across  its  outlet.  For  example,  the  rocky  coasts  of 
Labrador,  Newfoundland,  and  Cape  Breton  Island  protect  the 
outlets  of  the  St.  Lawrence  River  and  its  gulf  from  the  formation 
of  such  a  bar  as  that  which  forms  across  the  Columbia  River. 
Nature  sometimes  provides  a  trumpet-shaped  channel  with  its 
cross-section  proportioned  to  the  tidal  flow,  of  which  the  St. 
Lawrence  is  an  example. 

If  the  estuary  is  wide  and  its  banks  are  irregular,  the  main  river 
channel  may  be  tortuous  and  shifting,  with  depths  on  shoals  in- 
sufficient for  navigation.  Under  such  conditions  a  system  of 
longitudinal  dikes,  with  the  distance  between  dikes  reduced  in 
proportion  to  the  flow  of  the  tide,  will  reduce  the  periods  of  slack 
water  and  will  prevent  inequalities  of  flow  in  different  sections 
of  the  estuary.  This  is  of  benefit  to  navigation,  although  it  reduces 
the  volume  of  the  tidal  prism.  The  increase  in  channel  depths 
and  in  tidal  oscillation  will  compensate  in  great  measure,  however, 
for  the  resulting  reduction  in  widths.  Since  the  flow  at  mid-tide 
is  the  greatest  and  diminishes  at  both  high  and  low  water,  regulat- 
ing dikes  directing  and  concentrating  the  lower  part  of  the  ebb 
tide  may  be  sufficient  to  attain  the  depths  required  for  navigation, 
and  the  upper  portion  of  the  flood  tide  may  be  permitted  to  fill  the 
portion  of  the  estuary  behind  the  dikes  and  thus  prevent  the  reduc- 


130  RIVERS  AND   HARBORS 

tion  in  the  volume  of  the  tidal  prism.  But  if  the  channel  is  very 
tortuous  and  shifting,  high  dikes  are  necessary. 

The  reduction  of  the  volume  of  the  tidal  prism  is  objectionable 
where  an  obstructive  bar  forms  across  the  river's  mouth,  and  to 
make  the  diminution  as  little  as  practicable  it  has  been  proposed 
to  supplement  the  channel  formed  by  dikes  rising  to  mid-tide  by  a 
wider  channel  limited  by  dikes  rising  above  high  tide.  There  is 
a  practical  difficulty  of  construction,  however.  The  space  between 
the  two  systems  of  dikes  must  be  given  a  gradual  upward  slope 
from  the  low-water  dikes  to  the  high-water  dikes  or  there  will 
be  a  sudden  lateral  expansion  of  the  incoming  tide  above  mid- 
stage,  accompanied  by  eddy  action  detrimental  to  the  direct  tidal 
flow. 

The  spur-dike  which  is  employed  extensively  for  the  regulation 
of  non-tidal  rivers  is  not  so  well  adapted  to  the  improvement  of 
estuaries,  since  it  creates  eddies  and  also  a  retardation  or  accelera- 
tion of  the  tidal  flow  which  tends  to  deposit  material  temporarily 
in  suspension.  There  is,  moreover,  a  possibility  that  the  tidal  flow 
may  follow  the  eddy  current  around  the  end  of  such  a  dike  and 
adopt  a  tortuous  path  difficult  to  navigate  instead  of  preserving 
the  straight  course  desired,  besides  leaving  a  shoal  in  mid-channel. 
In  the  early  attempts  to  improve  the  navigation  of  the  Appomat- 
tox  River  in  Virginia,  its  channel  was  contracted  by  a  system 
of  spur-dikes  inclining  upstream  in  accordance  with  the  German 
system  of  improving  non-tidal  rivers.  A  sinuous  channel  re- 
sulted, with  deep  water  between  the  dikes  first  on  one  side  of 
the  river  and  then  on  the  other,  with  mid-channel  depths  less 
than  those  that  formerly  existed.  By  connecting  the  ends  of 
the  dikes  by  training  walls,  satisfactory  mid-channel  depths  were 
obtained. 

The  material  deposited  in  shoals  becomes  compacted  and  re- 
quires a  greater  force  to  place  it  in  motion  again  than  the  force 
which  originally  transported  it.  Hence  the  regulation  of  an 
estuary  is  greatly  facilitated  by  dredging.  In  a  non-tidal  river 
the  force  which  creates  a  bar  recurs  at  every  river  rise,  but  in  an 
estuary  when  the  channel  across  a  bar  is  once  enlarged,  the  cause 
of  the  deposition  is  removed  and  the  material  carried  in  suspension 
has  no  greater  tendency  to  deposit  at  that  locality  than  at  any 
other,  provided  the  dredged  channel  coincides  with  the  natural 
direction  of  flow  of  the  tidal  currents. 


ESTUARIES  131 

It  is  not  unusual  for  a  dredged  channel  to  enlarge  its  section 
without  the  aid  of  training  walls.  This  occurred  in  a  channel 
excavated  by  hydraulic  dredges  in  the  St.  Lawrence  River  below 
Montreal.  During  a  falling  river  the  dredged  channels  of  the 
Mississippi  frequently  increase  their  cross-section,  but  this  en- 
largement is  accompanied  by  a  fill  during  rising  stages. 

When  a  channel  is  so  designed  that  its  cross-section  increases  in 
proportion  to  the  increase  in  the  tidal  flow,  the  river  discharge 
aids  the  ebb  tide  in  maintaining  channel  depths.  However,  the 
necessities  of  navigation  frequently  require  a  channel  of  such  depth 
and  width  at  the  head  of  the  estuary  that  its  progressive  widening 
toward  its  mouth  is  impracticable,  due  to  the  topographical  features 
of  its  banks.  Thus  it  may  happen  that  a  channel  of  uniform  width 
must  be  constructed  because  a  widening  of  the  channel  could  be  at- 
tained only  by  the  removal  of  high  or  rocky  bluffs  at  great  expense. 
In  such  cases  the  duration  of  slack  water  is  increased  at  the  head 
of  the  estuary,  the  river  discharge  tends  to  deposit  its  sediment 
there,  and  periodic  dredging  is  then  required  to  remove  the  bars 
which  form. 

The  river  Clyde  (7)  is  a  conspicuous  example  of  the  successful 
application  of  proper  principles  to  the  improvement  of  tidal  rivers. 
In  its  natural  state  the  Clyde  was  an  insignificant  stream  connect- 
ing Glasgow  with  the  Firth  of  Clyde,  the  distance  from  Glasgow 
to  the  mouth  of  the  Clyde  at  Greenock  being  21  miles.  Its  low- 
water  depth  was  about  \\  feet  at  Glasgow  and  at  spring  tides  3J 
feet,  notwithstanding  the  fact  that  the  range  of  spring  tides  in  the 
Firth  of  Clyde  is  about  11  feet. 

The  original  project  for  its  improvement  contemplated  obtaining 
7  feet  of  water  up  to  Glasgow  at  high  water  of  neap  tides  and  was 
to  be  obtained  by  contracting  the  river  by  jetties  at  the  worst  bars. 
At  the  beginning  of  the  nineteenth  century  the  systematic  con- 
struction of  low  rubble  training  walls  was  undertaken  with  a  chan- 
nel width  between  them  of  180  feet  just  below  the  harbor  of  Glas- 
gow and  gradually  increasing  to  a  width  of  696  feet.  In  a  report 
submitted  in  1835  it  was  stated  that  the  depth  at  low  water  in  the 
harbor  at  that  time  was  from  7  to  8  feet,  and  15  feet  at  high  water 
of  spring  tides. 

The  growth  of  commerce  demanding  greater  channel  capacity, 
the  river  was  given  a  width  of  450  feet  in  Glasgow  harbor,  370  feet 
just  below  it,  and  gradually  increased  in  width  to  1000  feet  within 


132  RIVERS  AND   HARBORS 

6  miles  of  its  mouth.  The  effect  has  been  to  depress  the  low  water 
at  Glasgow  8  feet  and  increase  the  depth  at  spring  tides  to  30  feet. 
The  first  improvement  lowered  the  high  water  at  ordinary  spring 
tides  about  6  inches,  but  a  further  enlargement  of  the  channel  has 
raised  it  about  9  inches  so  that  at  present  it  is  from  2  to  3  inches 
higher  than  it  was  in  1758  (8). 


CHAPTER  XIII 
THE  MOUTHS   OF  RIVERS 

At  the  mouths  of  rivers,  ocean  waves  and  currents  are  en- 
countered which  are  caused  not  only  by  the  tides  but  also  by  winds. 
In  mid-ocean,  while  the  height  of  the  wave  produced  by  tidal 
action  has  been  computed  to  vary  from  0.73  foot  to  1.95  feet  (1), 
waves  over  40  feet  high  have  been  observed  during  storms.  As 
the  water  shoals  in  the  vicinity  of  a  coast,  the  tidal  wave  increases 
in  height  and  the  storm  wave  diminishes,  but  the  tidal  wave  along 
the  shore  is  also  markedly  increased  or  diminished  both  at  high 
water  and  low  water  by  the  action  of  the  wind,  depending  on  the 
direction  in  which  it  is  blowing.  It  is  to  be  noted,  however,  that 
while  the  storm  wave  breaks  in  shallow  water,  it  can  also  greatly 
increase  in  height  in  deep,  trumpet-shaped  bays  in  a  manner 
similar  to  the  tidal  wave. 

While  the  height  that  waves  attain  is  of  importance  in  the 
determination  of  the  amount  of  the  tidal  flow  entering  estuaries 
and  harbors  and  the  elevation  to  be  given  works  of  improvement, 
the  vital  elements  in  determining  the  stability  of  structures  ex- 
posed to  wave  action  are  their  oscillation  and  the  force  generated 
thereby. 

When  a  wind  blows  over  a  body  of  water,  it  imparts  to  the 
particles  on  its  surface  an  oscillatory  motion.  Where  the  depth 
is  great,  the  particles  of  water  at  the  surface  thus  set  in  motion 
move  in  circular  orbits,  whose  diameter  is  equal  to  the  height  of 
the  wave.  Hence  a  water  particle  moves  in  the  direction  of  the 
wind  in  the  upper  portion  of  its  path,  and  has  a  reverse  motion 
below.  The  particles  below  the  surface  acquire  a  similar  motion 
but  the  diameter  of  their  orbits  rapidly  diminishes  through  fric- 
tional  resistance,  and  the  motion  becomes  inappreciable  at  a  depth 
below  the  trough  of  the  wave  equal  to  its  height.  The  form  as- 
sumed by  the  water  surface  under  such  circumstances  is  trochoidal, 
and  this  trochoid  is  advancing  constantly  in  the  direction  of  the 
wind. 

When  such  a  wave  is  created  in  shoal  water,  the  bed  obstructs 

133 


134  RIVERS  AND   HARBORS 

the  reverse  motion  of  the  particles  and  their  orbit  becomes  ellip- 
tic. The  form  of  the  wave  becomes  more  nearly  cycloidal  and 
there  is  a  certain  depth  at  which  the  crest  of  the  wave  assumes  the 
cusp  shape  of  the  cycloid  instead  of  the  rounded  form  of  the 
trochoid. 

In  still  shoaler  water,  the  frictional  resistance  of  the  bed  re- 
tards the  reverse  flow  of  the  particles  to  such  an  extent  that  the 
particles  moving  in  the  upper  portions  of  their  orbits  cannot 
adjust  themselves  to  the  changed  conditions.  Then  the  wave 
breaks,  and  the  upper  portion  flows  as  an  auxiliary  wave  over  the 
lower  part. 

While  the  relation  of  the  height  to  the  length  of  a  wave  is  a 
function  of  the  duration  and  the  intensity  of  the  wind,  it  can  be 
assumed  for  purposes  of  illustration  that  a  wave  has  a  length  of 
about  25  times  its  height.  The  velocity  with  which  the  wave  is 
transmitted  varies  from  2  feet  per  second  to  over  100  feet  per 
second,  depending  on  the  intensity  of  the  wind.  It  is  evident 
that  a  body  such  as  a  breakwater,  opposing  such  a  moving  mass, 
is  exposed  to  great  pressure.  If  the  wave  breaks  just  before  it 
reaches  the  obstacle,  the  pressure  is  transformed  into  a  blow  of 
great  force,  since  water  is  incompressible.  Since  the  particles  of 
water  are  free  to  slide  over  one  another  when  their  velocity  is 
checked,  even  a  breaking  wave  converts  some  of  the  energy  of 
its  blow  into  friction  losses,  and  the  force  of  the  blow  cannot  be 
accurately  computed.  If  the  wave  breaks  at  a  considerable  dis- 
tance from  the  obstacle,  as  on  an  outer  bar,  its  force  is  dissipated 
and  the  bar  acts  as  a  protection  to  the  breakwater.  Dyna- 
mometer pressures  of  over  6000  Ibs.  per  square  foot  have  been  re- 
corded on  structures  exposed  to  breaking  waves.  The  accuracy 
of  the  records  has  been  questioned,  however. 

A  more  vivid  realization  of  the  force  of  waves  is  obtained  from 
the  damage  they  have  caused  to  breakwaters.  At  Peterhead  (2), 
blocks  of  concrete  weighing  40  tons  have  been  displaced  at  levels 
of  17  to  36  feet  below  low  water.  During  the  construction  of  the 
Plymouth  breakwater  blocks  of  stone  weighing  from  7  to  9  tons 
were  removed  from  the  sea  slope  at  the  level  of  low  water  and 
carried  over  the  top  a  distance  of  138  feet  and  deposited  on  the 
inside.  At  Wick,  two  stones  weighing  8  and  10  tons,  respectively, 
were  thrown  over  the  parapet  of  the  breakwater,  the  top  of  which 
was  21  feet  above  high  water,  and  blocks  of  concrete  weighing 


THE   MOUTHS   OF   RIVERS  135 

respectively  1350  and  2500  tons  were  displaced.  At  the  Ymuiden 
breakwater  a  block  of  concrete  weighing  20  tons  was  lifted  verti- 
cally by  a  wave  to  a  height  of  12  feet  and  landed  on  the  top 
of  the  pier  which  was  five  feet  above  high  water.  At  Bilboa 
a  solid  block  of  the  breakwater  weighing  1700  tons  was  over- 
turned (2). 

As  a  further  illustration  of  the  power  of  waves,  Mr.  Shields 
gives  examples  of  their  destructive  effect  on  rocky  cliffs.  Thus 
at  Wick,  at  a  height  of  from  70  to  80  feet  above  sea-level,  blocks 
of  stone  weighing  15  tons  have  been  detached  from  their  stratified 
beds,  lifted  over  ledges  7  feet  in  height,  and  driven  uphill  for  a 
distance  of  fully  100  feet  from  the  edge  of  the  cliff.  At  Holburn 
Head  on  the  coast  of  Scotland,  at  an  elevation  of  130  feet  above 
sea-level,  a  similar  degradation  of  the  cliff  is  occurring,  the  debris 
being  moved  over  80  feet  and  consisting  of  stones  weighing  up 
to  half  a  ton. 

From  these  illustrations  it  will  be  noted  that  when  a  wave  dashes 
against  a  vertical  obstruction,  a  column  of  water  is  projected  into 
the  air  to  a  great  height,  and  this  mass  falls  with  destructive  energy 
not  only  on  the  obstacle  which  formed  it  but  also  on  the  water, 
producing  a  blow  which  is  transmitted  to  the  foundation  of  the 
structure,  sometimes  with  disastrous  results. 

When  a  wave  breaks  perpendicularly  on  a  sloping  beach,  its 
upper  portion  is  driven  up  the  beach  until  its  energy  is  exhausted. 
The  water  then  recedes  with  increasing  velocity  until  it  is  met  by 
a  succeeding  wave,  when  it  continues  its  course  as  an  undertow, 
the  incoming  waters  flowing  over  it.  This  motion  acts  on  the 
material  of  which  the  beach  is  composed,  the  incoming  wave 
moving  material  inland  and  the  outgoing  flow  carrying  it  toward 
the  sea,  where  it  comes  to  rest  when  the  impetus  of  the  undertow 
is  exhausted.  It  results  that  with  waves  of  a  certain  height  and 
frequency,  the  beach  is  being  scoured  at  a  given  elevation  by  both 
the  inflow  and  outflow,  the  material  scoured  by  the  inflow  being 
deposited  at  the  upper  limits  of  the  water's  motion,  while  that 
moved  by  the  outflow  forms  an  outer  shoal.  The  portion  of  the 
beach  thus  acted  upon  will  assume  a  steep  slope. 

With  a  lowering  of  the  tide  or  a  reduction  in  wave  height,  this 
shoal  may  be  scoured  by  the  incoming  wave  and  its  material 
driven  toward  the  beach,  so  that  erosion  and  fill  are  alternately 
occurring  along  an  exposed  coast.  With  a  strong  onshore  wind 


136  RIVERS  AND  HARBORS 

the  beach  is  being  destroyed,  while  an  offshore  wind  tends  to  re- 
build it. 

When  the  waves  approach  the  coast  obliquely,  their  crests  after 
they  break  run  along  the  beach  and  their  waters  return  to  the  sea 
by  different  paths  than  their  paths  of  approach.  This  produces 
a  movement  of  the  material  scoured  along  the  shore  in  the  direc- 
tion the  wind  is  blowing,  which  is  increased  by  the  current  which 
is  at  the  same  time  created  in  the  water.  If  there  is  an  obstacle 
along  the  beach,  the  moving  sand  is  checked  by  it,  and  piles  up 
on  its  windward  side.  Beyond  it  erosion  still  continues,  and  as 
the  obstacle  prevents  the  replacement  of  the  material  excavated 
by  that  moving  along  the  shore,  the  beach  caves  more  rapidly 
beyond  an  obstacle  than  when  it  is  unobstructed. 

When  the  material  thus  transported  encounters  the  currents  at 
the  mouth  of  a  river  it  is  carried  up  the  estuary  during  certain 
portions  of  the  flood  tide  and  out  to  sea  during  a  part  of  the  ebb, 
but  during  certain  periods  of  high  and  low  water  the  material  is 
deposited,  forming  a  bar  across  the  river's  mouth,  which  also  may 
be  increased  by  material  transported  by  the  river's  discharge. 
The  tidal  ebb  and  flow  scours  a  channel  through  this  bar,  but  as 
it  is  constantly  being  increased  by  material  moved  along  the  shore, 
there  is  a  tendency  to  force  the  channel  from  its  normal  path  by 
continued  accretions  on  its  windward  side.  This  lateral  move- 
ment of  the  channel  continues-  until,  during  some  violent  storm  on 
the  ocean  or  a  flood  in  the  river,  the  river  currents  have  sufficient 
force  to  scour  through  the  bar  along  the  lines  of  natural  flow. 
The  old  channel  then  tends  to  fill  and  the  lateral  channel  movement 
is  again  repeated. 

On  sandy  shores  where  the  winds  have  a  prevailing  component 
moving  the  sand  in  one  direction,  this  lateral  channel  movement 
occurs  in  more  or  less  regular  cycles  at  an  unimproved  river  mouth, 
and  two  or  more  channels  will  be  in  existence  across  the  bar,  one 
enlarging  as  the  others  are  filling.  The  energy  of  the  tidal  flow 
will  be  dissipated  among  these  channels. 

A  knowledge  of  the  direction  of  the  prevailing  winds  is  of  great 
importance  in  planning  works  for  the  improvement  of  the  mouths 
of  rivers  and  harbors,  but  it  should  be  borne  in  mind  that  the  wind 
bloweth  where  it  listeth,  and  that  for  considerable  periods  the 
water  currents  may  flow  in  the  direction  opposite  to  that  of  the 
prevailing  winds,  and  that  these  currents  carry  their  load  of  ma- 


THE    MOUTHS    OF   RIVERS  137 

terial,  but  in  less  amounts,  which  is  also  deposited  across  the  mouth. 
The  character  of  the  shore,  whether  rocky  or  sandy,  and  its  ex- 
posure, which  determines  the  intensity  of  the  waves,  are  frequently 
of  more  importance  than  the  direction  of  the  prevailing  winds. 
Waves  with  a  long  fetch  which  impinge  on  a  sandy  shore, 
though  caused  by  only  an  occasional  wind,  may  move  more  material 
than  those  produced  by  prevailing  winds  of  even  greater  intensity 
which  strike  a  rocky  coast  or  whose  dimensions  are  reduced  by 
outlying  islands  or  other  obstructions.  The  ground  swell  is  an 
illustration  of  a  wave  of  great  force  which  is  independent  of 
local  winds.  It  is  created  by  storms  in  mid-ocean  and  is  propa- 
gated to  great  distances  as  a  long,  low  wave  of  great  depth.  Local 
winds  merely  produce  on  it  a  choppy  sea. 

In  the  improvement  of  the  mouths  of  rivers  three  cases  arise. 
First,  the  river's  discharge  may  be  the  determining  force  in  form- 
ing the  bar.  Second,  the  movement  of  material  along  the  shore 
may  be  the  important  element.  Third,  both  the  river's  sediment 
and  the  littoral  drift  may  combine  to  form  the  bar.  When  a  river 
with  a  large  discharge  flows  directly  into  a  sea  where  the  rise  and 
fall  of  the  tide  is  small,  the  sediment  which  it  carries  is  deposited 
at  its  mouth  and  a  bar  is  formed  which  gradually  extends  outward, 
through  which  the  river  forces  its  way  in  several  channels.  During 
flood  stages  the  discharge  of  the  river  deposits  sediment  on  these 
bars  above  the  sea-level  and  each  channel  becomes  a  mouth. 
This  deposit  of  sediment  is  termed  a  delta,  as  at  the  mouths  of 
the  Mississippi,  the  Nile,  the  Danube,  and  the  Rhone. 

But  there  may  exist,  also,  some  distance  in  front  of  the  mouth 
of  the  river,  a  drifting  sand  bar  parallel  to  the  coast  which  rises 
at  places  above  sea-level.  In  that  case,  the  river  deposits  its 
sediment  in  the  lagoon  thus  formed  and  its  discharge  enters  the 
sea  through  one  or  more  channels  across  this  bar.  The  rivers  of 
Texas  and  North  Carolina  have  mouths  of  this  character.  For- 
merly some  of  the  rivers  of  the  valley  of  the  Po  discharged  into 
the  Lagoon  of  Venice  in  a  similar  manner. 

Even  where  the  normal  fluctuation  of  the  tide  is  small,  as  in 
the  Gulf  of  Mexico  and  in  the  Adriatic  Sea,  the  tidal  flow  may  be 
so  increased  by  the  action  of  winds  that  the  area  of  these  lagoons 
becomes  an  important  element  in  determining  the  depths  across 
the  bars  at  their  outlets.  This  tidal  basin  may,  however,  extend 
inland  with  its  greatest  length  in  the  direction  of  the  river  dis- 


138  RIVERS  AND  HARBORS 

charge,  forming  a  tidal  estuary,  and  the  bar  may  be  the  result 
of  the  deposition  of  the  material  carried  along  the  coast  and  of 
that  discharged  by  the  river. 

When  a  river  discharges  through  several  mouths  its  energy  is 
dissipated,  and  it  is  incapable  of  maintaining  a  deep  channel 
through  any  of  them.  This  suggests  as  a  method  of  improvement 
the  concentration  of  the  flow  in  one  mouth  by  closing  the  side 
channels.  Such  a  method  would  be  temporarily  successful,  but 
it  would  concentrate  all  the  sediment  carried  by  the  river  in 
the  same  channel,  with  the  result  that  a  second  delta  would  begin 
to  form  at  this  new  outlet,  similar  to  the  one  nature  has  already 
created.  The  new  delta  would  have  numerous  mouths  which  in 
the  process  of  time  also  would  require  regulation. 

Even  when  the  tidal  fluctuations  are  small,  the  winds  along  the 
sea  coast  produce  currents  of  considerable  force  and  capable  of 
moving  large  amounts  of  the  light  silt  brought  down  by  a  river. 
If  the  prevailing  winds  have  a  decided  component  in  one  direc- 
tion along  the  coast,  a  littoral  current  is  produced,  which  creates 
a  steeper  seaward  slope  in  the  deposits  from  the  mouths  that  dis- 
charge at  right  angles  to  its  flow  than  in  those  which  enter  the 
sea  in  an  oblique  direction.  Hence,  if  that  mouth  is  selected  whose 
deposits  have  the  steepest  sea  slope  and  only  enough  discharge  is 
concentrated  in  it  to  insure  an  adequate  depth  of  water  across  the 
bar,  the  littoral  current  will  remove  a  large  amount  of  the  deposit 
and  will  reduce  the  rate  of  progress  of  the  bar  seaward,  while  the 
remaining  mouths  will  discharge  the  great  mass  of  water  and 
sediment  so  far  from  the  one  being  improved  that  its  bar  will 
not  be  increased  by  littoral  drift. 

For  these  reasons,  in  improving,  the  mouth  of  the  Danube  (3), 
the  Salina  Pass  was  selected,  although  it  discharged  less  than  8 
per  cent  of  the  river  flow,  and  at  the  mouth  of  the  Mississippi  (4) 
the  South  Pass  was  chosen  originally,  which  also  had  a  discharge 
of  about  8  per  cent.  In  its  original  condition  Salina  Pass  was 
about  300  feet  wide.  South  Pass  was  about  600  feet  wide.  The 
adopted  projects  contemplated  a  depth  across  the  bar  at  the 
mouth  of  the  Danube  River  of  20  feet,  and  at  the  mouth  of  the 
Mississippi  of  30  feet.  To  concentrate  the  river  flow  across  the 
bar,  the  banks  of  the  pass  were  extended  by  means  of  parallel 
jetties  to  water  of  the  depth  desired  beyond  it.  The  distance  be- 
tween jetties  at  the  Salina  Pass  is  600  feet.  The  jetties  at  the 


THE   MOUTHS   OF   RIVERS  139 

South  Pass  are  1000  feet  apart,  but  the  width  is  contracted  to 
600  feet  by  spur-dikes  200  feet  long  built  at  right  angles  to  them 
on  both  sides  of  the  channel.  In  both  instances  the  projected 
depths  have  been  obtained  and  the  movement  of  the  bar  seaward 
is  so  gradual  that  the  cost  of  the  periodic  extension  of  the  jetties 
necessary  to  maintain  the  required  depths  is  not  excessive.  At 
the  South  Pass  occasional  storms  may  so  interfere  with  the 
littoral  current  as  to  cause  a  temporary  shoaling,  and  dredging 
is  employed  to  restore  the  channel  and  to  obviate  the  delay  which 
would  result  from  relying  on  the  river  current  to  reestablish  it. 

The  improvement  of  the  channel  across  the  bar  has  been  ac- 
companied by  the  enlargement  of  the  South  Pass  its  entire  length 
and  an  increase  in  its  discharge  to  about  14  per  cent  of  that  of 
the  entire  river.  Although  dredging  was  necessary  originally  at 
the  head  of  the  pass  to  maintain  a  channel  thirty  feet  deep,  at  the 
present  time  a  submerged  sill  and  contraction  works  are  necessary 
to  limit  the  flow  into  it. 

The  improvement  at  the  mouth  of  the  Mississippi  has  caused  a 
rapid  increase  in  the  commerce  of  New  Orleans,  and  there  is  a 
resultant  demand  for  a  channel  of  35  feet  depth  to  the  Gulf. 
While  the  South  Pass  could  be  enlarged  readily  so  as  to  obtain 
the  increased  discharge  necessary  to  scour  such  a  channel  across 
the  bar,  the  soil  which  forms  the  banks  is  an  alluvium  readily 
scoured  by  river  currents  and  gulf  waves  and  there  was  danger 
that  its  further  enlargement  would  be  accompanied  during  floods 
by  breaks  through  the  narrow  necks  of  land  which  separate  it  from 
the  waters  of  the  Gulf.  To  prevent  such  a  catastrophe  it  would 
have  been  necessary  to  revet  this  pass  along  its  entire  length. 
The  Southwest  Pass  (5),  which  originally  carried  about  one-third  of 
the  discharge  of  the  river,  therefore  was  selected  in  the  project  of 
1902  for  obtaining  a  depth  of  35  feet  across  the  bar.  For  economic 
reasons  converging  dikes  about  5600  feet  apart  at  their  land-ends 
and  2800  feet  apart  at  the  bar  were  substituted  for  the  parallel 
jetties  constructed  at  the  South  Pass,  and  dredging  was  relied 
upon  to  maintain  the  channel  between  them.  This  dredging  was 
found  to  be  excessive,  and  in  1916  the  project  was  modified  by 
limiting  the  channel  width  uniformly  to  2400  feet  by  two  parallel 
interior  jetties.  This  modified  project  is  now  in  process  of  exe- 
cution. 

Whether  the  channel  will  maintain  itself  without  excessive 


140  RIVERS  AND   HARBORS 

dredging  when  thus  contracted,  can  be  determined  only  when  the 
work  is  completed.  Investigations  have  demonstrated  that  the 
much-discussed  Gulf  Stream  is  non-existent  at  the  mouths  of  the 
Mississippi  River  (6),  but  that  the  winds  produce  strong  littoral 
currents  which  are  capable  of  moving  large  amounts  of  silt  at 
both  outlets.  The  silt  which  flows  in  the  Southwest  Pass  is  also 
a  large  percentage  of  that  of  the  main  stream. 

Due  to  the  difference  in  length  of  the  two  passes,  it  is  feared 
that  there  will  be  difficulty  in  maintaining  the  proper  proportions 
of  the  river  discharge  in  them,  since  there  are  changes  of  slope  caused 
by  floods  in  the  river  and  by  tides  and  winds  in  the  gulf,  with  a 
tendency  to  deposit  sediment  in  one  or  the  other.  The  improve- 
ment of  two  mouths  of  a  river  is  an  experiment  without  precedent 
in  engineering  practice. 

At  the  mouth  of  the  Rhone  (7),  where  the  whole  discharge  was 
confined  to  a  single  outlet,  the  growth  of  the  bar  has  been  so 
rapid  that  the  extension  of  the  jetties  necessitates  an  excessive 
expenditure  for  maintenance,  and  a  lateral  canal  connecting  the 
river  with  some  harbor  beyond  the  zone  of  the  river's  deposits 
has  become  necessary  to  afford  a  navigable  outlet.  Such  a  canal 
usually  requires  a  lock  at  its  junction  with  the  river  to  provide 
for  variations  in  flood  heights,  with  the  consequent  delays  to 
navigation. 

Where  the  river  approaches  the  sea  through  an  estuary  with  a 
pronounced  tidal  flow  in  the  direction  of  the  river's  discharge  and 
gradually  enlarges  its  section  so  as  to  facilitate  the  tidal  flow,  the 
tides  assume  a  direction  and  force  that  will  maintain  a  deep  channel 
at  the  river's  mouth.  The  Thames,  the  Potomac,  and  the  James 
rivers  are  of  this  class.  Their  outlets  maintain  themselves  with- 
out auxiliary  works.  In  outlets  of  this  character;  local  currents 
sometimes  form  bars  which  can  be  permanently  removed  by  dredg- 
ing, as  at  the  entrance  to  New  York  Bay.  The  coasts  of  New 
Jersey  and  Long  Island  form  the  trumpet-shaped  outlet  con- 
sidered so  essential  to  create  an  ample  tidal  flow  in  an  estuary, 
but  the  littoral  and  tidal  currents  meeting  between  Sandy  Hook 
and  Coney  Island  create  a  complicated  local  flow  which  produces 
a  bar  extending  across  the  estuary  between  them,  with  several 
channels  across  it.  One  of  these  near  the  Long  Island  shore  has 
been  dredged  to  a  depth  of  20  feet  and  a  width  of  600  feet,  a 
second  one  near  Sandy  Hook  to  a  depth  of  30  feet  and  a  width  of 


THE   MOUTHS   OF   RIVERS  141 

1000  feet,  and  an  intermediate  channel,  called  the  Ambrose 
Channel,  to  a  depth  of  40  feet  and  a  width  of  2000  feet.  The  two 
deeper  channels  have  been  maintained  with  little  dredging,  and 
the  Ambrose  Channel  has  enlarged  its  section  materially  since  it 
has  been  excavated.  The  conditions  which  permit  the  main- 
tenance of  two  deep  channels  at  this  harbor  are  exceptional  and 
are  probably  due  to  a  natural  diversion  of  the  tidal  currents,  one 
of  which  passes  from  the  lower  bay  into  the  upper  bay  and  thence 
to  the  Hudson  and  East  rivers,  while  a  second  passes  east  of 
Staten  Island  to  Newark  Bay  and  the  rivers  that  empty  into  it  (8) . 

It  frequently  happens  that  topographical  features  prevent  the 
gradual  enlargement  of  the  section  of  the  estuary,  and  that  there 
is  a  littoral  drift  of  material  through  which  the  tidal  currents  are 
unable  under  natural  conditions  to  create  and  maintain  a  channel 
of  suitable  depth  for  navigation.  It  then  becomes  necessary  to 
contract  the  outlet  and  concentrate  the  tidal  flow  in  a  single 
channel  of  reduced  width,  which  will  maintain  the  required  depth 
across  the  bar. 

Parallel  jetties  have  been  used  extensively  for  this  purpose, 
as  at  Calais,  Dunkirk,  and  Ostende  (9).  Since  these  harbors  have 
been  maintained  for  several  hundred  years  they  afford  good  illus- 
trations of  the  ultimate  effect  of  such  structures  on  the  movement 
of  littoral  drift.  These  harbors  were  located  originally  near  the 
outlets  of  small  streams,  with  tidal  basins  not  proportioned  to 
the  discharge  of  the  river.  The  outlets  were  obstructed  by  bars 
with  insignificant  channels  over  them  at  low  tide.  With  the 
growth  of  commerce  deeper  channels  were  required,  and  parallel 
jetties  were  constructed  across  the  bar  for  the  purpose  of  inducing 
tidal  scour.  A  deeper  channel  was  created  by  this  means,  but 
in  process  of  time  the  littoral  drift  piled  the  sand  against  these 
jetties  and  the  entire  shore  line  was  built  out  to  their  ends.  The 
bar  then  reformed  in  advance  of  its  original  position  and  the  in- 
flowing tides  also  brought  material  into  the  harbors  which  gradually 
filled  them.  This  action  frequently  was  accelerated  by  the  recla- 
mation by  the  inhabitants  of  land  covered  at  low  tides. 

The  reduction  of  the  tidal  area  diminished  the  force  of  the  out- 
flowing tides,  and  a  further  extension  of  the  jetties  was  unable  to 
maintain  a  satisfactory  channel  across  the  bar.  To  increase  the 
tidal  outflow  sluicing-basins  were  then  constructed,  which  con- 
sisted of  reservoirs  which  were  filled  by  the  inflowing  tide,  the 


142  RIVERS  AND  HARBORS 

water  retained  by  gates,  and  then  suddenly  released  when  it 
would  have  the  greatest  scouring  effect.  By  these  means  a  narrow 
channel  has  been  maintained  at  these  ports,  which  can  be  utilized 
by  small  channel  vessels,  but  is  inadequate  for  the  large  modern 
steamships. 

At  the  Malamocca  entrance  (10)  to  the  Lagoon  of  Venice  parallel 
jetties  have  successfully  maintained  a  channel  of  30  feet  depth  at 
low  tides.  The  littoral  drift  is  small  and  the  effect  of  the  jetties 
is  to  concentrate  the  outflow  from  the  lagoon  and  prevent  its 
conflict  with  the  littoral  current  until  deep  water  is  attained. 
The  range  of  tides  in  the  Adriatic  Sea  only  slightly  exceeds  that 
of  the  Gulf  of  Mexico,  but  the  Lagoon  has  been  gradually  filling, 
both  from  material  carried  in  from  the  sea  and  from  that  deposited 
by  the  discharge  of  the  rivers  of  the  Po  Valley  that  formerly  flowed 
into  it.  To  reduce  this  fill,  the  waters  of  the  Brenta  River  have 
been  diverted  from  the  Lagoon. 

A  more  successful  means  of  concentrating  the  tidal  flow  across 
the  bar  is  by  means  of  converging  dikes,  as  at  Charleston  and 
Galveston  (11).  As  their  name  implies,  such  dikes  starting  from 
the  shore,  gradually  reduce  the  width  of  the  waterway  toward 
the  crest  of  the  bar.  They  add  the  area  of  the  bar  which  they 
enclose  to  the  natural  tidal  capacity  of  the  basin,  thus  forming  a 
sluicing  basin  properly  located  to  increase  the  flow  at  their  ex- 
tremities. The  littoral  drift,  instead  of  causing  an  advance  of  the 
foreshore,  is  guided  by  the  outer  surface  of  these  dikes  into  deep 
water,  where  a  littoral  current  can  carry  it  beyond  the  entrance 
or  the  tidal  inflow  and  outflow  can  hold  it  in  suspension  until  it  is 
deposited  in  deep  water.  For  the  successful  accomplishment  of 
works  of  this  character,  a  rapid  construction  is  necessary  and  a 
channel  should  be  opened  through  the  bar  by  dredging  so  that 
the  tidal  flow  can  operate  at  full  depth,  since  tidal  action  extends 
to  the  river's  bed  while  wave  action  frequently  is  of  limited  depth. 
If  reliance  is  placed  on  the  dikes  alone  to  scour  the  channel  across 
the  bar,  there  is  a  tendency  to  push  the  bar  further  out  to  sea  with- 
out attaining  sufficient  depth  over  it  to  insure  a  proper  tidal  flow. 
Occasionally  the  littoral  drift  is  so  pronounced  in  one  direction, 
that  only  a  single  jetty  is  required  to  maintain  the  channel.  At 
the  mouth  of  the  Cape  Fear  River  (12),  the  southern  trend  of  the 
drift  has  formed  a  long  bar  which  creates  a  sheltered  bay  behind 
it.  There  is  only  a  small  flow  of  sand  in  a  northerly  direction 


THE   MOUTHS   OF   RIVERS  143 

along  the  coast.  By  closing  the  channel  across  this  bar  called 
the  New  Inlet  and  preventing  others  from  forming,  the  outlet 
into  the  bay  is  successfully  maintained  by  dredging. 

On  Lake  Erie,  the  entrance  to  Sandusky  Bay  has  been  improved 
by  a  single  jetty,  but  under  very  exceptional  conditions.  The 
tidal  flow  of  the  Great  Lakes  is  insignificant,  but  their  level  is 
affected  by  winds.  The  prevailing  winds  of  Lake  Erie  have  a 
westerly  component  which  raises  the  water  level  at  the  eastern 
extremity  of  the  lake  and  similarly  depresses  it  at  the  western 
end.  The  axis  of  Sandusky  Bay  is  parallel  to  that  of  the  Lake 
and  its  outlet  is  at  its  eastern  end,  while  the  bay  is  located  near 
the  western  extremity  of  the  lake.  A  westerly  wind,  therefore, 
depresses  the  water  in  the  lake  at  the  outlet  of  the  bay  while  it 
raises  the  water  of  the  bay  at  the  same  locality,  thus  creating 
a  head  which  generates  a  strong  current  along  the  directing  jetty. 
When  the  wind  subsides,  the  return  flow  into  the  bay  is  spread 
over  a  large  area  and  has  little  scouring  effect.  Moreover,  the 
littoral  drift  is  small. 

It  also  has  been  proposed  to  improve  the  outlets  of  tidal  estu- 
aries and  bays,  by  means  of  a  single  detached  jetty  called  a  re- 
action breakwater  (13),  and  in  addition  to  make  the  jetty  concave 
to  the  tidal  flow  so  as  to  increase  the  deepening  of  the  bar  by  the 
centrifugal  force  generated  when  water  is  forced  into  a  curved 
path.  Such  a  jetty  would  prevent  the  lateral  movement  of  the 
channel,  but  it  would  largely  increase  the  flow  of  sand  into  the 
harbor  instead  of  reducing  it.  If  located  on  the  opposite  side  of 
the  harbor  from  that  along  which  the  great  mass  of  littoral  drift 
is  moving,  as  at  Sandusky  Bay,  the  only  resistance  to  the  inflow 
of  the  littoral  drift  is  the  tidal  flow.  During  a  large  part  of  the 
flood  tide  there  would  be  a  flow  of  sediment  across  the  windward 
bar  into  the  harbor,  which  would  be  removed  only  partially  by 
the  flow  concentrated  along  the  concave  dike  during  the  last  half 
of  the  ebb  tide.  If  located  on  the  windward  side,  there  is  a  move- 
ment of  sand  into  the  harbor  between  the  detached  breakwater 
and  the  beach,  sometimes  sufficient  to  scour  an  auxiliary  channel. 

The  concave  form  of  the  dike  also  would  have  an  injurious 
effect  on  the  tidal  flow.  As  explained  in  Chapter  III,  whenever 
water  is  forced  to  follow  a  curved  path,  there  is  generated  a  cen- 
trifugal force,  which  causes  the  filaments  of  water  to  move  in 
helicoidal  lines.  This  creates  a  local  scouring  effect  that  tends 


144  RIVERS  AND   HARBORS 

to  undermine  the  structure  which  opposes  the  rectilinear  flow 
and  produces  a  deep  narrow  channel  close  to  the  dike.  When 
the  water  passes  beyond  the  curved  obstruction,  this  helicoidal 
flow  diminishes,  and  the  additional  sediment  which  it  has  caused 
the  water  to  carry  is  deposited  in  a  bar  extending  obliquely  across 
the  channel,  this  action  creating  a  shoaling  above  the  dike  during 
the  flood  tide  and  beyond  the  dike  during  the  ebb.  The  curvi- 
linear path  thus  started  has  a  tendency  to  propagate  itself  along 
the  estuary,  first  impinging  on  one  bank  and  then  on  the  other, 
and  creating  a  shoal  at  every  crossing  by  the  change  from  heli- 
coidal to  rectilinear  motion. 

The  structures  constructed  at  the  mouths  of  rivers  are  not 
usually  exposed  to  the  intense  wave  action  which  harbor  break- 
waters have  to  resist,  since  the  littoral  drift  forms  a  sloping  beach 
on  which  the  waves  break  and  exhaust  their  force  before  reaching 
the  dikes.  Sand  sometimes  is  piled  against  a  jetty  to  the  height 
of  high  water  (14)  and  becomes  a  deciding  factor  in  determining 
the  elevation  to  be  given  the  structure. 

At  the  mouths  of  non-tidal  rivers,  the  deposit  is  frequently  a 
fine  silt.  If  rip-rap  is  deposited  directly  on  this  silt,  there  will  be 
an  excessive  settlement.  To  prevent  this  action  heavy  willow 
mats  are  sunk  as  foundation  courses  for  the  jetties,  the  rip-rap 
structures  being  placed  thereon.  At  the  outer  ends  of  the  jetties, 
which  are  exposed  to  a  stronger  wave  action,  large  concrete  blocks 
usually  are  substituted  for  the  rip-rap,  as  in  breakwater  construc- 
tion described  in  the  following  chapter. 

In  the  United  States,  mattresses  are  frequently  employed  as 
foundation  courses  on  bars  formed  by  littoral  drift.  This  has 
been  done  at  Charleston  and  at  Galveston.  In  such  cases,  how- 
ever, it  is  necessary  to  cover  the  mats  rapidly  with  a  layer  of  rip- 
rap in  order  to  induce  a  deposit  of  sand  and  thus  to  protect  the 
mats  from  the  ravages  of  the  teredo. 


CHAPTER  XIV 
HARBORS 

Many  harbors  are  located  near  the  mouths  of  rivers,  and  their 
entrances  and  the  protection  afforded  to  shipping  within  them 
are  dependent  on  the  works  necessary  to  maintain  the  river's 
outlet.  When  harbors  are  on  bays  or  on  slight  indentations  in 
the  coast  line,  however,  breakwaters  can  be  constructed  to  afford 
additional  protection.  The  Standard  Dictionary  defines  a  harbor 
as'  "a  sheltered  place  where  ships  can  find  protection  from  storms," 
which  definition  indicates  the  two  most  essential  elements  of  har- 
bor construction:  first,  protection  of  vessels  within  it,  and  second, 
a  safe  entrance  during  storms.  Where  a  harbor  is  not  merely 
one  of  refuge,  but  is  also  used  for  commercial  purposes,  a  third 
element  consists  of  suitable  arrangements  for  loading  and  un- 
loading the  cargoes  which  seek  the  port. 

To  afford  protection,  .the  breakwaters  should  intercept  the 
waves  formed  in  the  open  sea  over  an  area  which  will  accom- 
modate the  vessels  seeking  shelter  during  storms;  and  to  insure  a 
safe  entrance,  a  vessel  fleeing  before  a  gale  must  be  able  to  enter 
the  harbor  before  she  turns.  If  a  ship  has  to  maneuver  in  the 
open  sea,  there  is  danger  of  capsizing  or  drifting  on  to  the  piers 
which  limit  the  width  of  the  entrance.  The  entrance  therefore 
should  face  in  the  direction  of  the  greatest  storms.  While 
this  allows  a  vessel  to  enter  the  harbor  readily,  a  wave  can  follow 
the  same  path,  so  that  a  considerable  space  is  required  inside  the 
entrance  in  which  the  wave  can  expand  and  reduce  its  height  be- 
fore reaching  the  portion  of  the  harbor  in  which  shipping  is  moored. 

Stevenson's  formula  (1)  for  the  heights  of  waves  in  harbors  is 


in  which  X  is  the  height  of  wave  at  any  observed  point  in  the 
harbor,  H  is  the  height  of  the  wave  at  the  entrance,  b  is  the  width 
of  entrance,  D  is  the  distance  of  point  of  observation  from  en- 
trance, and  B  is  the  width  of  the  harbor  at  the  observed  point,  all 

145 


146  RIVERS  AND   HARBORS 

given  in  feet.  It  is  apparent  from  this  formula  that  the  height 
of  a  wave  in  a  harbor  is  a  function  of  the  width  of  the  entrance, 
and  of  the  angle  which  the  two  arms  of  the  breakwater  which  form 
it  make  with  each  other. 

In  a  discussion  of  the  proper  form  to  be  given  to  harbors  on  the 
Great  Lakes  to  give  the  greatest  tranquillity,  a  board  of  Engineer 
Officers  (2)  says:  "The  breakwaters  should,  theoretically,  in  order 
to  afford  the  greatest  protection  for  the  least  length,  be  given  a 
direction  perpendicular  to  the  line  of  approach  of  the  waves  caused 
by  the  heaviest  and  most  objectionable  storms,  i.e.  parallel  to 
the  waves  of  such  storms,  but  they  should  also  fulfill  the  con- 
dition that  the  entrance  between  them  be  in  a  depth  of  water 
which  will  allow  the  largest  storm  waves  to  pass  without  break- 
ing. They  should  make  with  each  other  an  angle  large  enough 
to  permit  sufficient  expansion  of  the  entering  wave,  and  they 
should  be  far  enough  apart  to  allow  a  vessel  to  enter  in  the  worst 
weather  without  danger  of  striking  on  either  side." 

The  first  consideration  in  determining  the  width  of  the  entrance 
is  the  contraction  necessary  to  give  the  desired  depth,  where  the 
tidal  basin  is  small.  Where  there  is  a  large  estuary,  the  first  con- 
sideration is  the  necessity  of  permitting  as  large  a  tidal  inflow  as 
possible  for  the  improvement  of  the  upper  sections  of  the  estuary. 
The  next  consideration  is  the  size  of  the  vessels  using  the  harbor. 
Finally,  the  disturbance  caused  by  the  inflowing  wave  must  be 
reduced  to  a  minimum. 

A  small  tidal  basin  whose  bar  has  been  deepened  by  parallel 
jetties  cannot  be  entered  safely  during  on-shore  winds,  nor  is  there 
adequate  protection  unless  interior  basins  are  constructed  to  afford 
additional  shelter.  When  the  harbor  has  been  improved  by  con- 
verging dikes,  the  area  thus  added  to  its  basin  frequently  renders 
an  interior  basin  unnecessary. 

On  the  Great  Lakes  most  of  the  harbors  originally  were  formed 
by  deepening  the  outlet  of  a  river  or  of  a  lake  by  means  of  parallel 
jetties,  and  to  prevent  the  destructive  inflow  of  waves  during 
storms  and  to  reduce  their  force,  it  has  been  necessary  to  construct 
breakwaters  in  the  lake  in  front  of  the  jetties.  Frequently,  as  at 
Buffalo  (3),  Cleveland  (4),  and  Chicago  (5),  the  growth  of  com- 
merce has  been  so  great  that  these  breakwaters  have  been  de- 
veloped into  outer  harbors.  At  many  of  the  minor  ports  the 
breakwaters  have  been  constructed  for  the  purpose  of  creating 


HARBORS  147 

a  basin  in  which  the  waves  can  flatten  out  before  reaching  the 
jetties,  and  of  enabling  a  vessel  to  enter  the  harbor  with  greater 
safety  during  storms.  They  also  permit  a  wider  entrance  to  be 
built  in  a  turbulent  sea.  Moreover,  a  channel  which  affords 
ample  depth  for  navigation  in  calm  weather  has  its  depth  greatly 
reduced  as  the  trough  of  a  large  wave  is  propelled  along  it.  In 
shallow  entrances  the  wave  may  break. 

On  the  Great  Lakes,  harbor  entrances  designed  for  vessels  of 
19  feet  draft  have  a  minimum  width  of  400  feet  in  water  over  30 
feet  deep.  After  entering  the  outer  basin,  the  wave  is  reduced 
sufficiently  so  that  the  boat  is  usually  able  to  continue  its  course 
in  a  channel  21  feet  deep  and  200  feet  wide. 

For  vessels  of  30  feet  draft  the  writer  has  recommended  an  en- 
trance at  least  600  feet  wide  (6).  With  a  properly  proportioned 
harbor  of  350  acres,  the  incoming  wave  soon  dissipates  its  force, 
so  that  a  ten-foot  wave  at  the  entrance  will  not  injuriously  affect 
shipping  moored  at  wharves  directly  in  front  of  it.  When  the 
area  of  the  enclosed  harbor  is  greater,  the  width  of  entrance  can 
be  safely  increased  to  800  or  1000  feet,  and  it  is  rarely  necessary 
to  limit  the  tidal  inflow  of  rivers  to  insure  a  quiet  harbor.  Some- 
times in  large  harbors  two  entrances  facing  in  different  directions 
can  be  provided  so  as  to  afford  additional  security  to  vessels  en- 
tering them.  This  has  been  done  at  Colombo  and  at  Boulogne. 
At  Citte  an  outer  breakwater  has  been  constructed  opposite  the 
entrance  and  facility  of  entrance  has  been  sacrificed  for  the  tran- 
quillity in  the  harbor. 

Experience  has  demonstrated  that  if  the  harbor  expands  sym- 
metrically on  both  sides  of  the  entrance,  Stevenson's  formula  for 
the  height  of  waves  gives  satisfactory  results,  but  in  a  harbor 
where  the  main  channel  follows  one  of  the  dikes  while  the  other 
merely  prevents  an  inflow  of  sediment,  a  wave  that  enters  can 
strike  the  channel  dike  obliquely  and  follow  along  it  with  little 
diminution  of  force.  If,  however,  there  is  a  break  in  the  con- 
tinuity of  the  dike  and  if  a  small  basin  with  a  sloping  beach  is 
formed  behind  it,  the  restrained  wave  expands  into  the  basin, 
and  its  height  is  greatly  reduced.  Such  stilling  basins  have  been 
constructed  at  Dieppe  (7)  and  at  Havre  (8). 

In  Chapter  XIII  illustrations  were  given  of  the  force  generated 
by  wave  action.  Since  the  breakwaters  employed  in  harbor  con- 
struction frequently  are  located  so  as  to  receive  the  full  force  of 


148  RIVERS  AND  HARBORS 

the  greatest  wave  created  in  the  sea  to  which  they  are  exposed, 
their  construction  is  extremely  difficult.  The  difficulty  is  in- 
creased by  the  great  depths  in  which  it  is  sometimes  necessary 
to  place  them. 

Two  methods  of  resisting  the  force  of  the  waves  have  been  em- 
ployed: by  a  rubble  mound,  and  by  a  vertical  wall.  These  two 
methods  also  have  been  combined  by  erecting  a  vertical  wall  as 
a  superstructure  on  a  foundation  of  rip-rap.  In  the  rubble  mound 
breakwater,  the  rubble  or  concrete  blocks  of  which  it  is  composed 
are  deposited  on  the  site  of  the  work,  and  are  permitted  to  assume 
such  slopes  as  the  waves  create.  These  slopes  are  functions  of 
the  intensity  of  the  wave  action  and  of  the  dimensions  of  the 
rubble.  Under  ordinary  conditions,  wave  action  extends  only  to 
a  depth  below  the  trough  of  the  wave,  equal  to  the  height  of  the 
wave.  Hence  the  mound  assumes  a  natural  slope  not  exceeding 
1  to  1 1  at  from  ten  to  twenty-five  feet  below  low  water,  de- 
pendent on  the  exposure.  In  the  breakwaters  of  this  class  which 
were  first  constructed,  the  rubble  deposited  was  the  run  of  the 
quarry  in  sizes  up  to  five  tons  in  weight,  and  above  the  level  of 
no  disturbance  the  slope  assumed  by  the  mound  at  exposed  sites 
was  about  1  to  10.  Such  was  the  case  at  Cherbourg,  France,  and 
at  Table  Bay,  Cape  of  Good  Hope  (7).  Even  where  there  was 
partial  protection,  as  at  Portland  (7),  slopes  of  1  to  6  were  obtained. 

Two  methods  have  been  employed  to  increase  the  sea  slope 
and  thus  diminish  the  dimensions  of  the  breakwater.  The  first 
method  consists  in  paving  the  exposed  face.  At  Plymouth  (7), 
England,  the  breakwater  above  low  water  was  given  a  sea  slope 
of  1  on  5  and  was  paved  with  granite  blocks  set  in  cement.  At 
Kingstown  (7),  a  pavement  of  granite  blocks  placed  on  edge  covers 
the  entire  surface  exposed  to  the  action  of  the  waves.  Portions  of 
the  breakwater  at  Buffalo,  N.  Y.,  were  constructed  in  this  manner. 

A  second  method  of  protecting  the  sea  slope  is  by  increasing 
the  dimensions  of  the  rubble  deposited  on  the  sea  face.  When 
rubble  of  sufficient  size  is  not  readily  obtained,  cc  icrete  blocks  are 
substituted.  Large  rubble  was  adopted  at  the  Delaware  Break- 
water (8),  at  San  Pedro,  Cal.,  at  Cleveland,  O.,  and  at  numerous 
other  harbors  on  the  Great  Lakes.  At  Port  Said,  Algiers,  and 
Alexandria  in  the  Mediterranean  Sea,  and  at  Osaka  in  Japan, 
concrete  blocks  have  been  substituted  for  rubble.  At  Port  Said  (7) 
the  main  portion  of  the  breakwater  consists  of  20-ton  concrete 


HARBORS  149 

blocks,  deposited  at  the  slope  they  naturally  assume.  At  Osaka 
(9),  the  body  of  the  breakwater  consists  of  small  rubble  overlaid 
with  concrete  blocks  weighing  about  4  tons  each,  on  a  sea  slope  of 
about  1  to  2.  This  harbor  is  exposed  only  to  moderate  seas, 
however. 

In  the  United  States,  the  width  of  the  crowns  of  breakwaters  is 
from  8  to  14  feet  on  the  Great  Lakes,  and  15  to  20  feet  when  ex- 
posed to  ocean  storms.  The  sea  slopes  are  1  to  3  to  a  depth  of 
from  12  to  25  feet  below  low  water,  and  below  those  depths  the 
natural  slope,  which  the  mound  assumes.  On  the  harbor  face  a 
slope  of  1  to  1.33  is  usually  adopted.  The  elevation  of  the  break- 
water on  the  Great  Lakes  rarely  exceeds  six  feet  above  high  water. 
At  the  Delaware  breakwater  it  is  14  feet  above  low  water,  at 
Sandy  Bay  22  feet  above  low  water. 

In  breakwater  construction  the  character  of  the  quarry  affects 
to  a  great  extent  the  methods  employed.  If  the  rock  on  blasting 
naturally  breaks  into  masses  more  or  less  rectangular,  the  body 
of  the  breakwater  can  be  composed  of  the  quarry  refuse,  and  the 
larger  pieces  can  be  laid  with  their  longer  dimensions  at  right 
angles  to  its  surface,  thus  forming  a  pavement  of  great  resisting 
power  to  wave  action.  If  the  rock  breaks  in  irregular  fragments, 
however,  they  cannot  be  bonded  together,  and  reliance  must  be 
placed  on  the  weight  of  the  individual  pieces  to  hold  them  in 
place.  Such  breakwaters  have  not  the  finished  appearance  of 
those  which  are  paved,  but  possess  the  advantage  that  they  break 
more  effectively  the  wave  which  impinges  against  them. 

If  the  pavement  is  smooth,  a  considerable  amount  of  water 
will  roll  up  its  surface  and  may  enter  the  harbor  over  the  top  of 
the  breakwater  in  sufficient  amounts  to  affect  its  tranquillity, 
and  undermine  the  harbor  face  of  the  breakwater.  If  the  sea 
slope  is  rough,  the  crest  of  the  wave  is  converted  into  spray,  cov- 
ering the  breakwater  with  a  foam,  but  having  little  effect  on  the 
harbor.1  To  prevent  their  displacement  the  individual  stones 
must  be  of  greater  size  for  the  same  slope  than  where  the  surface 
is  paved. 

'This  was  forcibly  illustrated  during  a  typhoon  at  Manila,  where  one  of  the  old  monitors 
with  a  low  free-board  moored  behind  the  detached  breakwater,  which  is  of  the  rough 
rubble  mound  type,  with  an  elevation  of  its  crest  of  but  five  feet  above  high  water.  The  spray 
created  by  the  waves  dashing  against  the  breakwater  was  sufficient  to  obscure  the  view  of  the 
monitor  from  shore,  but  the  writer  was  informed  by  the  Commander  of  the  vessel  that  the 
water  in  which  she  floated  had  only  a  slight  wave  motion  from  the  waves  passing  through  the 
harbor  entrance. 


150  RIVERS  AND   HARBORS 

Where  the  depth  is  great,  the  area  of  the  cross-section  of  a  rubble 
mound  breakwater  is  large,  and  a  considerable  reduction  in  its 
dimensions  and  therefore  of  the  quantity  of  material  required  in 
its  construction  can  be  obtained  by  the  substitution  of  a  break- 
water with  vertical  masonry  walls.  When  the  area  of  the  harbor 
is  limited,  such  a  substitution  may  be  necessary,  but  it  is  rarely 
economical  compared  with  the  use  of  large  facing  stones  for  the 
rubble  mound,  on  account  of  the  excessive  cost  of  underwater 
masonry  construction.  In  the  United  States,  the  vertical  type 
of  breakwater  has  been  employed  only  in  the  Great  Lakes,  where 
the  absence  of  the  teredo  and  other  sea  worms  has  permitted  the 
substitution  of  timber  cribs  for  masonry  walls. 

The  vertical  breakwaters  first  built  consisted  of  two  masonry 
walls  with  a  hearting  of  rubble.  Below  water  the  joints  were 
usually  not  filled  with  mortar.  The  oscillation  of  the  water  sur- 
face caused  by  the  impinging  waves  transmitted  a  pressure  through 
the  interstices  of  the  walls  to  the  water  in  the  hearting,  and  as  the 
wave  receded,  a  sufficient  interior  pressure  would  be  created  to 
force  some  of  the  stone  from  the  face  of  the  wall.  The  breach 
thus  formed  would  rapidly  enlarge,  and  the  hearting  would  escape 
through  it. 

This  interior  pressure  was  sometimes  sufficient  to  disrupt  a 
pavement  placed  on  top  of  a  breakwater  to  protect  it  from  the 
blow  caused  by  the  water  flowing  over  it  during  storms.  The 
overflowing  water  at  the  Wicks  breakwater  had  sufficient  force  to 
undermine  and  overturn  the  harbor  wall,  the  rubble  hearting  was 
then  washed  away,  and  the  sea  wall  collapsed  (10). 

By  replacing  the  rubble  hearting  by  masonry  so  that  the  break- 
water is  practically  a  monolith,  these  sources  of  danger  can  be 
avoided.  In  recent  breakwater  construction,  concrete  has  been 
generally  substituted  for  rubble  masonry.  The  portion  below  low 
water  is  formed  of  concrete  deposited  in  bags  to  form  a  monolith; 
between  high  and  low  water,  the  breakwater  is  built  of  concrete 
blocks  mortised  and  toggled  so  as  to  form  a  compact  mass;  and 
above  high  water  the  concrete  is  deposited  in  mass.  The  north 
breakwater  at  Aberdeen  is  composed  of  concrete  deposited  in  bags 
to  low  water,  and  of  mass  concrete  above  low  water  (7). 

The  pressure  which  is  exerted  horizontally  on  the  face  of  a 
breakwater  also  is  transmitted  vertically  to  the  sea  bed.  If  the 
face  is  vertical,  a  heavy  rip-rap  protection  is  required  to  prevent 


HARBORS  151 

scour  even  at  great  depths.  With  a  rubble  mound  the  waves 
expend  a  considerable  portion  of  their  force  running  up  and  down 
the  slope  as  on  a  beach,  and  the  blows  they  can  deliver  on  the  in- 
clined surface  tend  to  consolidate  the  mass,  particularly  if  it  is 
paved.  Against  a  vertical  face  in  deep  water  the  force  exerted 
is  one  of  pressure.  However,  if  a  vertical  superstructure  is  super- 
imposed on  a  rubble  mound  which  is  high  enough  to  break  the 
waves,  the  pressure  is  converted  into  a  blow  of  great  magnitude, 
which  will  hurl  a  column  of  water  into  the  air  to  great  heights. 
Such  a  force  is  very  difficult  to  control. 

The  Alderney  Breakwater  is  an  example  of  this  type  which  has 
failed  (11).  At  the  Sandy  Beach  Harbor  of  Refuge  (12),  Cape 
Ann,  Mass.,  the  superstructure  was  composed  of  stone  blocks 
laid  in  steps  to  a  slope  of  1  on  2,  the  height  of  the  steps  varying 
from  3J  to  5  feet,  but  with  the  same  result  as  at  Alderney.  The 
suction  which  accompanies  the  receding  waves  scoured  the  rubble 
mound  to  great  depths,  undermined  the  superstructure,  and  so 
loosened  the  stone  forming  it  that  masses  weighing  20  tons  were 
carried  on  to  the  sea  slope  of  the  mound. 

When  it  is  necessary  to  place  a  superstructure  on  a  rubble 
mound,  to  insure  permanency  the  mound  should  be  extended 
above  high  water  by  large  blocks  of  stone  or  concrete  to  form  a 
wave  breaker,  as  at  Ymuiden  at  the  entrance  to  the  Amsterdam 
Ship  Canal. 

In  many  European  harbors,  the  vertical  type  of  breakwater  is 
required,  so  that  the  harbor  side  of  the  structure  can  be  used  as 
a  quay  for  loading  and  unloading  vessels.  A  high  parapet  is  then 
necessary  on  the  sea  wall  to  enable  operations  to  be  carried  on 
during  storms.  Such  a  parapet  increases  the  force  of  the  wave 
against  the  structure  and  tends  to  cause  an  unequal  settlement  of 
the  foundations.  In  the  United  States,  vessels  rarely  moor  along 
breakwaters  except  at  certain  harbors  of  refuge  on  the  Great 
Lakes.  On  this  account  as  well  as  on  account  of  the  smaller 
height  of  tides,  breakwaters  having  a  lower  elevation  than  those 
of  Europe  prevail. 

When  breakwater  construction  was  first  undertaken  on  the  Great 
Lakes,  timber  was  cheap  and  a  very  economical  vertical  type  of 
breakwater  (13)  was  evolved,  consisting  of  timber  cribs  filled  with 
small  rip-rap,  which  was  covered  by  a  timber  flooring.  In  the 
fresh  waters  of  the  lakes,  timber  is  preserved  from  decay  and 


152  RIVERS  AND   HARBORS 

destruction  from  boring  insects  and  worms,  so  that  below  low 
water,  when  properly  built,  the  structure  proved  permanent,  and 
above  water  it  could  be  replaced  economically  as  it  decayed. 
Masses  of  ice  striking  against  the  timber  would  crush  it,  however, 
and  it  was  found  necessary  to  face  the  timber  on  the  lake  side  with 
sheet  iron  along  the  low  water  line  to  prevent  such  action.  The 
cross  braces  originally  were  dovetailed  into  the  main  timbers,  but 
if  the  dovetail  was  not  perfect,  water  would  find  its  way  through 
any  imperfection  as  the  waves  rose  and  fell,  and  in  process  of  time 
so  erode  the  joint  as  to  destroy  the  connection.  Long  iron  rods 
with  washers  and  bolts  were  then  added  to  the  structure.  In 
recent  years  the  high  price  of  lumber  has  rendered  a  concrete 
superstructure  more  economical.  At  Milwaukee  (14),  cribs  of  re- 
inforced concrete  have  been  substituted  for  timber  cribs  in  the 
substructure. 

In  the  arrangements  for  loading  and  unloading  vessels,  there  is 
a  radical  difference  between  European  practice  and  that  in  the 
United  States.  The  great  heights  which  the  tides  attain  around 
the  British  Islands  and  in  the  North  Sea  originally  were  utilized 
by  the  vessels  in  entering  harbors,  the  depths  being  insufficient 
during  low  water  for  them  to  cross  the  bars,  and  as  the  banks  of 
a  river  have  a  gradual  slope,  the  vessel  would  ground  at  low  water 
if  moored  close  to  the  bank,  or  would  require  lighters  to  transfer 
its  cargo  to  the  bank  if  anchored  in  midstream. 

To  overcome  this  difficulty  basins  are  constructed  in  which 
the  vessels  can  enter  at  high  tide.  Their  entrances  can  be  closed 
during  low  water  by  means  of  gates  so  as  to  maintain  the  water 
level  within  them  at  that  of  high  tide.  These  tidal  basins  enable 
vessels  to  be  moored  along  the  quay  walls  which  formed  them, 
and  their  cargoes  to  be  removed  by  the  ship's  tackle,  without 
recourse  to  lightering. 

In  the  harbors  along  the  Mediterranean  Sea,  the  tidal  oscilla- 
tion is  not  sufficient  to  necessitate  such  basins,  but  the  harbors 
originally  designed  were  of  small  area,  and  the  disturbance  caused 
by  incoming  waves  during  storms  was  therefore  so  great  that 
inner  basins  without  gates  were  required  to  protect  the  vessels 
while  discharging  cargo.  In  the  United  States,  the  moderate 
tidal  range  permits  vessels  to  cross  the  harbor  bars  during  a  much 
longer  period  than  at  corresponding  ports  in  the  north  of  Europe, 
and  tidal  basins,  open  only  at  high  tides,  would  delay  the  departure 


HARBORS  153 

of  vessels.  A  sufficient  depth  for  the  vessels  to  float  at  low  tide 
while  being  unloaded  usually  can  be  obtained  more  economically 
by  constructing  piers  into  deep  water  than  by  excavating  basins 
of  sufficient  depth  in  the  banks,  so  that  in  the  United  States 
pier  construction  or  a  quay  wall  parallel  to  the  channel  very  gen- 
erally replaces  the  tidal  basin  of  northern  Europe  or  the  enclosed 
basins  of  the  Mediterranean  Sea. 

Since  the  piers  extend  from  the  bank  into  the  channel,  the  space 
at  which  vessels  can  moor  along  a  given  frontage  is  largely  in- 
creased. The  water  area  of  the  harbor  is  correspondingly  re- 
duced, however,  and  the  tidal  flow  is  obstructed.  Some  relief 
is  afforded  by  supporting  the  superstructure  of  the  piers  on  piles 
which  permit  a  restricted  flow  between  them.  In  rivers  carrying 
a  large  amount  of  sediment  in  suspension,  the  piles  have  an  effect 
similar  to  permeable  dikes,  and  cause  a  deposit  not  only  under  the 
piers,  but  also  in  the  slips  between  them.  Such  deposits  are  so 
great  in  the  Mississippi  River  at  New  Orleans,  that  not  only  is 
dredging  required  annually,  but  also  sluicing  devices  have  to  be 
maintained  to  remove  the  deposits  under  the  piers.  Unless  such 
deposits  are  flushed  out  before  low  stages  occur  in  the  river,  large 
masses  have  a  tendency  to  slide  into  the  channel  carrying  the 
supporting  piles  of  the  dock  with  them,  and  thus  destroying  it. 

The  piers  in  New  York  harbor  have  been  lengthened,  as  the 
dimensions  of  vessels  have  increased  during  recent  years,  and 
thereby  the  space  in  the  river  channel  of  the  Hudson  River 
frontage,  which  is  required  for  maneuvering  boats,  has  become 
so  restricted  that  the  Federal  government  has  intervened  and 
limited  the  length  of  piers. 

Harbor  and  dock  lines  also  are  imposed  at  numerous  other 
localities. 


CHAPTER  XV 
THE  ECONOMIES   OF  WATER  TRANSPORTATION 

One  of  the  most  important  problems  that  has  to  be  solved  by  an 
engineer  in  charge  of  the  improvement  of  rivers  and  harbors  is  the 
determination  of  whether  a  given  project  is  worthy  of  being  un- 
dertaken. 

While  this  question  in  the  United  States  is  one  for  Congressional 
action  rather  than  executive  decision,  Congress  has  delegated  its 
powers  on  this  question  to  a  certain  extent  to  the  Corps  of  En- 
gineers of  the  Army.  It  requires  a  report  from  Engineers  of  the 
Corps  on  the  worthiness  of  a  project  for  river  or  harbor  improve- 
ment, before  it  takes  final  legislative  action.  A  knowledge  of  the 
relative  economies  of  transportation  by  land  and  by  water  is 
therefore  requisite  for  all  officers  in  charge  of  river  and  harbor 
districts. 

Before  the  employment  of  steam  to  replace  animal  traction  as 
a  means  of  propulsion,  the  river  afforded  such  economies  in  the 
transportation  of  products  as  to  eliminate  competition  by  land 
vehicles.  A  team  of  horses  can  haul  200  tons  of  freight  in  a  canal 
boat  with  the  same  exertion  it  requires  to  move  ten  tons  on  a 
railroad  track  or  one  ton  on  an  ordinary  dirt  road.  As  a  result, 
in  Europe  the  early  centers  of  trade  and  manufacture  were  in- 
variably located  on  rivers,  and  the  growth  and  development  of 
the  country  has  been  along  its  waterways. 

The  invention  of  the  locomotive,  however,  disturbed  this  re- 
lation. While  it  requires  the  same  tractive  force  to  move  the 
200-ton  canal  boat,  the  10-ton  car,  or  the  1-ton  wagon  at  two  miles 
per  hour  whether  obtained  from  a  team  of  horses  or  from  a  steam 
engine,  when  the  speed  is  increased  to  20  miles  an  hour,  the  re- 
sistance of  the  boat  to  motion  through  the  water  enormously 
increases,  as  compared  with  the  friction  on  the  rail.  This  is  par- 
ticularly the  case  in  contracted  waterways. 

The  boat  in  its  motion  is  continually  displacing  a  mass  of  water 
equal  to  that  of  its  weight,  and  the  amount  of  water  displaced 

154 


ECONOMIES   OF  WATER  TRANSPORTATION  155 

per  hour  is  proportional  to  the  velocity.  By  giving  a  proper  shape 
to  the  boat,  the  force  necessary  to  displace  the  water  can  be  re- 
duced, but  it  is  also  necessary  to  have  a  sufficient  space  around 
it  to  permit  the  water  to  flow  readily  from  in  front  of  the  boat  to 
the  vacuum  created  behind  it.  In  a  shoal,  narrow  channel  this 
space  becomes  so  contracted  that  there  is  created  a  rising  of  the 
water  surface  in  advance  of  the  boat  and  a  lowering  behind  it. 
The  water  cannot  flow  with  sufficient  velocity  around  the  boat  to 
maintain  the  water  level  and  the  boat  is  compelled  to  move  not 
only  the  water  it  displaces,  but  a  large  percentage  of  the  contents 
of  the  channel  in  front  of  it,  in  the  form  of  a  wave  of  considerable 
height,  and  the  water  surface  may  be  so  lowered  at  the  boat's 
stern  as  to  cause  it  to  rub  the  river  bed. 

A  limit  is  soon  attained,  therefore,  beyond  which  it  is  uneco- 
nomical to  increase  the  speed.  This  limit  varies  with  the  width 
and  the  depth  of  the  channel,  the  form  of  the  barge,  and  the 
methods  employed  in  combining  the  barges  in  tows.  For  pur- 
poses of  the  present  argument  let  the  limit  be  assumed  to  be  five 
miles  per  hour  for  such  rivers  as  the  Ohio  and  the  Mississippi. 
For  small  canals  it  will  be  less,  and  for  vessels  navigating  the  Great 
Lakes  or  the  ocean  it  may  reach  twelve  or  fifteen  miles  per  hour. 

The  resistance  to  be  overcome  in  moving  a  car  over  a  railroad 
track  is  also  a  variable  function  of  the  speed,  the  curvature,  and 
the  grades.  For  a  boat,  a  small  increase  in  speed  above  the 
economic  limit  necessitates  so  large  an  increment  of  power  as  to 
make  the  consumption  of  fuel  prohibitive.  The  same  increase 
in  the  speed  of  a  railroad  car  will  increase  only  slightly  the  fuel 
bill  for  the  locomotive.  Since  it  requires  the  same  crew  to  handle 
a  freight  train  whether  the  average  speed  is  5  miles  per  hour  or 
20  miles  per  hour,  the  saving  in  the  men's  wages  resulting  from  a 
quicker  trip  may  compensate  for  the  extra  coal  consumed,  and  the 
cost  per  ton-mile  for  hauling  freight  in  a  train  which  travels  at 
an  average  speed  of  20  miles  per  hour  may  be  less  than  the  cost 
when  it  moves  at  a  smaller  average  speed.  Hence  the  steam 
locomotive,  by  increasing  the  speed  with  which  cars  are  moved, 
has  rendered  the  cost  of  transportation  on  land  more  nearly  equal 
to  that  by  water  than  it  was  when  animals  were  the  chief  means 
of  traction. 

It  has  been  found  also  that  the  force  required  to  move  a  ton  of 
freight  and  the  work  expended  per  ton-mile  are  diminished  as  the 


156  RIVERS   AND   HARBORS 

load  in  the  car  is  increased.  The  substitution  of  a  car  which 
carries  50  tons  of  freight  for  one  whose  load  is  20  tons,  reduces  the 
traction  per  ton  of  car  and  contents  on  a  level  track  approximately 
from  7.6  pounds  to  3  pounds  for  a  velocity  of  5  miles  per  hour  (1). 
Thus  four  freight  cars  carrying  fifty  tons  each  require  much  less 
force  to  propel  them  than  twenty  cars  containing  ten  tons  each. 
The  increase  in  the  size  of  the  freight  car  has  enabled  the  loco- 
motive to  haul  200  tons  of  freight  at  the  rate  of  5  miles  per  hour 
with  the  same  application  of  force  that  was  required  on  the  canal 
boat  also  carrying  200  tons  and  moving  in  the  contracted  waters 
of  a  canal  at  the  rate  of  2  miles  per  hour.  By  increasing  the  ca- 
pacity of  the  locomotive,  a  large  number  of  cars  can  be  hauled  in 
the  same  train,  and  train  loads  of  from  2500  tons  to  3000  tons 
are  regularly  hauled  at  the  present  time  by  one  locomotive  and  a 
train  crew  of  five  men,  thereby  producing  great  economies  in  the 
labor  cost  per  ton-mile  of  freight  carried.  Where  no  progress  has 
been  made  in  the  methods  of  transportation  by  water,  the  loco- 
motive has  forced  the  old-fashioned  canal  boat  to  suspend  business. 

By  enlarging  the  boat,  however,  equally  great  economies  can 
be  introduced  into  water  transportation  as  the  enlarged  car  has 
produced  for  the  railroads,  and  wherever  water  transportation 
successfully  competes  with  transportation  by  rail  it  will  be  found 
that  there  has  been  a  large  increase  in  the  dimensions  of  the 
vessels  employed.  Thus  on  the  Great  Lakes  the  freighters  of 
from  12,000  to  15,000  tons  capacity  have  been  substituted  for 
those  of  2000  tons,  and  on  the  Rhine  barges  of  a  capacity  of 
1500  to  2000  tons  are  now  used  in  place  of  the  former  200-ton 
canal  boats.  The  enlarged  boat,  however,  requires  a  channel  of 
greater  area,  to  reduce  the  resistance  to  motion  through  the  water. 
If  it  is  necessary  to  construct  a  canal  for  the  water  transportation, 
it  has  been  found  that  the  necessary  enlargement  of  section  will 
increase  its  cost  so  much  that  the  interest  on  the  investment  and 
the  cost  of  maintenance  will  exceed  the  saving  produced  by  the  in- 
crease in  size  of  the  boats,  unless  a  very  large  traffic  is  created  (2). 

In  an  improved  river  this  is  not  the  case,  as  the  width  of  the 
cross-section  is  usually  very  large  compared  with  that  of  the  boat. 

On  the  ocean,  a  vessel  is  exposed  to  severe  strains  due  to  wave 
action  during  storms  and  must  be  constructed  with  sufficient 
structural  strength  to  resist  them.  This  requires  certain  relations 
between  the  length,  width,  and  draft  of  the  vessel.  The  ocean 


ECONOMIES   OF  WATER  TRANSPORTATION          157 

freighter  with  a  carrying  capacity  of  about  12,000  tons  has  a 
draft  of  about  30  feet.  On  the  Great  Lakes,  the  intensity  of  the 
wave  action  is  not  so  great,  and  sufficient  structural  strength  can 
be  obtained  for  a  vessel  of  12,000  tons  capacity  if  it  has  a  draft 
of  21  feet.  On  a  river,  wave  action  during  storms  is  insignificant, 
and  a  satisfactory  barge  of  2000  tons  capacity  can  be  constructed 
with  a  draft  of  but  8  feet.  By  uniting  six  such  barges  in  a  tow,  a 
carrying  capacity  of  12,000  tons  is  readily  attained. 

In  the  economical  production  of  steam  power,  the  compound  or 
triple-expansion  condensing  marine  engine  has  a  great  advantage 
over  the  high-pressure  non-condensing  locomotive.  Theoretically, 
therefore,  it  would  seem  that  transportation  by  water  still  should 
be  more  economical  than  by  rail,  provided  the  improvements  which 
have  been  made  in  boat  construction  are  utilized  properly. 

A  comparison  of  the  records  of  the  latest  types  of  locomotives 
when  hauling  heavy  trains  on  the  standard  railroads  of  the  country 
with  those  of  steamboats  and  barges  on  the  Great  Lakes  and  on 
the  rivers  of  the  United  States  confirms  this  expectation  (3).  The 
fuel  economy  of  the  modern  freighter  on  the  Great  Lakes  far  ex- 
ceeds that  of  the  railroad  locomotive  even  with  the  most  level 
track  and  with  the  least  curves.  On  rivers  that  have  a  navigable 
depth  at  low  water  of  6  feet,  by  assembling  a  number  of  barges 
in  tows,  similar  economic  results  are  attained.  The  amount  of 
human  labor  per  ton-mile  of  freight  carried  is  also  less  for  the 
lake  boat  or  the  tow  of  barges  than  for  the  train.  However, 
where  navigation  is  limited  to  the  river  packet,  which  was  the 
prevailing  method  of  river  transportation  forty  years  ago,  the 
recent  improvements  in  the  locomotive  and  car,  and  the  reduction 
of  grades  and  curves  in  the  track,  have  rendered  rail  transportation 
cheaper  in  both  the  elements  of  fuel  consumption  and  of  expenditure 
of  human  labor  per  ton-mile  of  freight  carried. 

These,  however,  are  frequently  minor  factors  in  the  total  cost 
of  transportation.  The  annual  interest  on  the  cost  of  constructing 
the  rail  bed,  or  of  deepening  the  river,  of  constructing  the  requisite 
number  of  locomotives  and  cars,  or  boats,  the  annual  outlay  for 
maintenance,  the  overhead  charges  for  supervision,  insurance,  etc., 
may  largely  exceed  the  cost  of  fuel  and  labor  when  the  traffic  is 
relatively  small. 

In  many  of  these  items  the  waterway  has  an  advantage  over  the 
railroad.  It  is  pertinent  therefore  to  inquire  why  the  water- 


158  RIVER'S  AND   HARBORS 

borne  commerce  on  certain  rivers  has  diminished  in  recent  years, 
while  rail  transportation  has  enjoyed  an  enormous  growth. 

It  is  often  assumed  that  the  decline  in  water  transportation  has 
been  caused  by  underhand  methods  on  the  part  of  railroad  ad- 
ministrations, but  it  is  unquestionable  that  there  are  certain 
fundamental  principles  (neglected  by  those  engaged  in  water 
transportation)  which  have  enabled  the  railroad  managements 
successfully  to  meet  water  competition.  The  owners  of  steam- 
boats have  failed  to  give  the  regularity  of  service  which  commerce 
demands.  During  the  period  of  the  year  when  the  business  of  a 
town  was  not  active  they  have  neglected  it  and  sought  traffic 
elsewhere.  A  train  runs  on  a  regular  schedule.  The  railroad 
official  could  therefore  make  a  contract  with  a  shipper  in  wnich 
he  guaranteed  regularity  of  service  on  condition  that  the  railroad 
would  also  be  used  when  the  boats  returned  to  the  regular  routes. 
The  rebate  formerly  could  be  used  to  insure  the  execution  of  the 
contract. 

Another  great  advantage  which  the  railroad  formerly  possessed 
was  due  to  through  bills  of  lading.  A  steamboat  can  deliver 
freight  only  at  water  points,  and  its  transportation  to  localities 
inland  must  be  completed  by  rail  or  wagon.  The  railroad  running 
from  the  river  town  nearest  to  the  point  of  ultimate  destination 
honored  only  a  local  bill  of  lading  from  the  river  town  when  mer- 
chandise was  shipped  by  water,  while  it  accepted  a  through  bill  of 
lading  from  the  origin  of  shipment  to  the  point  of  ultimate  des- 
tination from  a  connecting  railroad.  The  shipper  naturally  se- 
lected a  rail  route  instead  of  a  water  route  under  such  conditions. 
Rates  for  short  distances  which  were  relatively  higher  than  through 
rates  afforded  merely  an  additional  reason  for  this  preference. 

The  fact  that  the  steamboat  is  confined  to  a  waterway  also 
compels  the  loading  or  unloading  of  its  cargo  on  the  river  bank. 
When  a  warehouse  or  a  factory  is  located  inland,  the  truck  which 
transports  the  merchandise  to  and  from  the  river  also  charges 
local  rates  for  short  distances  which  are  relatively  higher  than 
through  rates.  A  track  can  be  laid  wherever  a  road  bed  of  suit- 
able grades  and  curvature  can  be  constructed.  The  railroad 
official  by  constructing  a  switch  into  the  warehouse  or  factory 
eliminates  not  only  this  expensive  truck  haul,  but  also  the  ex- 
penditure of  labor  which  is  necessary  in  order  to  transfer  the 
freight  between  the  dock  and  the  vessel. 


ECONOMIES   OF  WATER  TRANSPORTATION  159 

However,  when  the  ship  owner  and  the  railroad  manager  work 
in  harmony  and  there  is  a  large  traffic,  the  terminal  charges  can 
be  reduced  to  a  minimum  by  the  introduction  of  mechanical  ap- 
pliances. Cheap  terminal  facilities  have  been  as  large  a  factor  in 
developing  the  commerce  of  the  Great  Lakes  and  reducing  the 
freight  rates  as  the  improvement  of  the  vessels  which  are  employed 
in  transporting  the  cargoes.  When  a  railroad  owns  the  boat  line, 
it  can  substitute  a  car-ferry  for  an  ordinary  freighter,  transporting 
both  the  car  and  its  contents  together.  Then  terminal  charges  are 
eliminated,  and  the  cost  per  ton  mile  of  revenue-producing  freight 
transported  is  greatly  reduced,  notwithstanding  the  non-revenue- 
producing  weight  of  the  car  which  is  transported  by  the  boat. 

Car-ferries  are  extensively  used  as  substitutes  for  bridges  and 
tunnels  in  transporting  rail  freight  across  rivers,  lakes,  and  bays. 
Moreover,  in  New  York,  they  afford  an  economical  method  of  dis- 
tributing freight  to  different  sections  of  the  harbor.  The  Pennsyl- 
vania Railroad  and  the  Hudson  and  Manhattan  tunnels  have 
affected  only  passenger  travel,  and  the  car-ferry  is  still  used  by 
rail  lines  for  distributing  freight  to  the  different  boroughs  of  the 
city  as  well  as  to  some  of  the  ocean  steamship  terminals. 

The  most  vital  principle  which  has  led  to  the  growth  of  rail 
traffic,  and  which  has  hindered  transportation  by  water  is  one, 
however,  that  the  vessel  owner  cannot  control.  It  is  the  necessity 
of  finding  a  market  at  the  point  of  ultimate  destination  in  which 
the  freight  can  be  sold  at  a  profit.  The  United  States  owes  its 
expansion  to  its  railroads,  and  its  development  has  been  across  its 
rivers  instead  of  in  the  direction  of  their  flow.  The  western  farmer 
or  miner  seeks  a  market  for  his  products  in  an  eastern  city  and  the 
wholesale  merchant  in  New  York  or  Boston  finds  his  best  cus- 
tomers in  the  west.  Most  of  the  rivers  of  the  country  cross  the 
lines  of  traffic  thus  created,  instead  of  running  parallel  to  them. 
On  many  rivers  of  the  United  States,  the  boat  is  relegated,  there- 
fore, to  the  minor  function  of  collecting  the  products  of  agricul- 
ture and  mining  along  the  banks,  and  transporting  them  to  a  rail- 
road, whose  terminal  is  a  large  city.  The  cost  of  the  transfer  of 
freight  by  manual  labor  between  car  and  boat  is  so  great,  more- 
over, that  a  waterway  that  extends  along  a  main  line  of  commerce 
must  exceed  at  least  200  miles  in  length  to  permit  its  economical 
utilization  for  through  freight,  if  the  boat  cargoes  are  received 
from  cars  and  delivered  to  them.  The  difference  in  cost  between 


160  RIVERS   AND   HARBORS 

the  transportation  over  the  rail  or  in  the  waterway  is  exceeded  by 
the  terminal  charges  when  the  boats  travel  only  a  short  distance. 

There  is  also  a  certain  density  of  traffic  necessary  for  the  suc- 
cessful operation  of  a  line  of  transportation.  If  a  river  flows 
through  a  thinly  settled  country,  with  no  mines  along  its  bank, 
and  few  factories,  there  exists  comparatively  little  traffic  from 
which  to  build  up  a  large  system  of  transportation. 

The  ability  of  railroads  to  divert  traffic  from  the  waterways 
has  been  greatly  restricted  recently.  By  the  decisions  of  the 
Interstate  Commerce  Commission,  discriminations  between  ship- 
pers have  been  prohibited.  By  recent  Congressional  enactments  the 
railroads  are  required  to  grant  not  only  the  same  through  bills  of 
lading  to  steamboat  lines  as  to  railroads,  but  also  to  similarly  pro- 
rate the  revenues  from  through  freight.  Hence,  a  waterway  ex- 
tending along  a  natural  line  of  communication  is  not  limited  to 
local  freight,  but  can  compete  for  through  traffic.  The  cities  and 
towns  on  the  river  banks  have  awakened  also  to  the  value  of 
efficient  terminal  facilities,  and  economical  methods  of  loading 
and  unloading  boats  have  been  introduced  in  many  localities. 
The  southern  ports,  especially  New  Orleans  and  Galveston,  are 
rapidly  increasing  both  their  exports  and  imports.  This  should 
create  a  greater  north  and  south  movement  of  freight  by  water. 
While  the  influence  of  the  Panama  Canal  possibly  has  been 
exaggerated,  it  should  give  an  additional  tendency  to  a  north  and 
south  transportation  of  products,  and  the  movement  of  the  center 
of  population  of  the  country  westward  increases  the  cost  of  rail 
transportation  to  and  from  the  Atlantic  Coast.  Moreover, 
there  has  been  a  remarkable  manufacturing  development  in  the 
southern  states  in  recent  years  which  should  affect  the  movement 
of  freight  and  increase  the  density  of  traffic. 

In  determining  the  worthiness  of  a  waterway  project,  the  en- 
gineer is  required  to  take  into  consideration  not  only  existing 
commerce  but  also  reasonably  prospective  commerce.  While  he 
should  not  permit  himself  to  be  unduly  influenced  by  visionary 
schemes  of  the  future  development  of  the  country,  he  should  be 
aware  of  the  natural  tendencies  of  its  growth,  and  should  give 
due  weight  to  those  which  will  expedite  it. 

A  cheap  north  and  south  line  of  transportation  across  the 
United  States  is  rapidly  becoming  an  economic  necessity.  It  is 
self-evident  that  the  Mississippi  Valley  affords  the  cheapest  line. 


ECONOMIES   OF  WATER  TRANSPORTATION  161 

Its  natural  terminals  are  at  Chicago  on  the  Great  Lakes,  which 
is  the  center  of  manufacture  and  trade  of  the  western  portion  of 
the  United  States,  and  at  New  Orleans  on  the  Gulf  of  Mexico, 
which  is  the  natural  port  for  the  importation  and  exportation  of 
the  products  required  and  raised  in  the  Middle  West.  The  tribu- 
taries of  the  Mississippi  River  afford  numerous  auxiliary  feeders  for 
such  a  route,  some  of  which  could  readily  become  main  lines  of 
transportation.  For  example,  Minneapolis  and  Kansas  City  are 
as  important  centers  of  the  grain  trade  as  is  Chicago,  and  the 
Pittsburgh  district  far  exceeds  it  as  a  center  of  iron  and  steel 
production. 

Inadequate  terminal  facilities  also  largely  increase  the  cost  of 
water  transportation  on  account  of  the  delays  in  loading  and 
in  unloading  the  boats.  With  the  exception  of  the  bill  for  fuel, 
the  expenses  of  operating  a  vessel  are  the  same  whether  she  is 
in  motion  or  tied  to  a  dock,  but  her  receipts  are  a  function  of  the 
number  of  trips  she  can  make  in  a  season  with  a  full  cargo.  If 
she  spends  a  large  portion  of  her  time  in  port,  the  wages  of  her 
crew,  the  interest  on  the  investment,  insurance  and  deterioration, 
continue,  while  the  revenues  received  diminish.  Moreover,  there 
is  a  large  saving,  if  there  is  a  well-balanced  shipment  of  freight, 
so  that  she  can  transport  full  cargoes  in  both  directions. 

M.  Galliot  in  a  recent  paper,  in  the  ANN  ALES  DBS  FONTS  ET 
CHAUSSEES  (2),  analyzes  the  various  elements  that  enter  into  the 
cost  of  transportation,  and  deduces  mathematical  equations  which 
express  the  relations  between  them.  The  deduction  can  be  made 
from  his  analysis  that  in  a  canal  the  dimensions  of  a  boat  are 
limited  economically  by  the  cost  of  the  enlargement  of  the  water- 
way; that  in  a  river  the  dimensions  of  the  units  of  a  tow  are  de- 
termined economically  by  the  depth  of  the  river  channel;  that  in 
the  ocean  there  are  theoretically  no  limits;  the  greater  the  carry- 
ing capacity  of  the  vessel,  the  more  economical  is  the  transporta- 
tion per  ton  of  capacity,  as  long  as  the  vessel  remains  at  sea.  As 
soon  as  a  vessel  enters  a  harbor,  however,  the  savings  resulting 
en  voyage  are  diminished  by  delays  due  to  the  lack  of  port  fa- 
cilities. 

The  12,000-ton  freighter  of  the  Great  Lakes  is  economical,  not 
only  on  account  of  her  size,  but  also  because  the  time  which  she 
spends  in  port  has  been  reduced  to  a  minimum.  Mechanical  ap- 
pliances enable  the  vessel  to  be  loaded  with  coal  or  iron  ore  in  a 


162  RIVERS  AND   HARBORS 

few  hours,  and  unloaded  in  less  than  a  day.  The  distance  of  ap- 
proximately 900  miles  between  harbors  on  Lake  Superior  and  on 
Lake  Erie  permits  her  to  remain  in  port  only  a  small  percentage 
of  her  time. 

If  a  similar  vessel  is  employed  in  transporting  freight  between 
minor  ports  on  the  Atlantic  Ocean,  in  which  the  terminal  facilities 
are  such  that  as  many  days  are  expended  in  transferring  freight 
between  boat  and  dock  as  hours  are  required  for  the  same  purpose 
on  the  Great  Lakes,  the  time  in  port  becomes  excessive.  If  three 
vessels  of  4000-ton  capacity  are  substituted  for  the  12,000-ton 
freighter,  they  can  be  loaded  and  unloaded  at  separate  docks 
simultaneously  and  the  port  delays  reduced  thereby. 

During  a  certain  period  in  the  development  of  lake  traffic,  the 
whaleback  and  its  consorts  were  employed  very  generally;  but 
a  cargo  can  be  propelled  through  water  of  ample  depth  more 
rapidly  and  more  economically  in  a  single  vessel  than  in  a  tow; 
and  coincident  with  the  enlargement  of  unloading  devices  at  the 
various  lake  ports,  large  single  vessels  have  replaced  barge  navi- 
gation, while  barge  tows  still  prevail  along  the  Atlantic  coast.  In 
river  navigation,  an  advantage  in  employing  barges  of  moderate 
capacity  results  from  the  facility  with  which  cargoes  destined  for 
different  city  landings  can  be  loaded  in  separate  barges",  and  the 
proper  barge  can  be  left  at  each  port  to  be  unloaded  and  loaded, 
while  the  remainder  of  the  tow  continues  its  voyage,  as  a  car  is 
switched  out  of  a  freight  train  at  a  way  station  of  a  railroad. 

Density  of  traffic  has  also  an  important  influence  on  port  de- 
velopment. In  order  to  be  profitable,  the  2500-ton  train  re- 
quires a  large  movement  of  freight,  and  such  trains  are  operated 
only  between  important  terminals  in  large  railway  systems.  On 
the  branch  lines  trains  of  less  capacity  must  be  employed.  The 
same  principle  applies  to  water  transportation.  The  25,000-ton 
freighter  cannot  be  operated  profitably  when  the  sources  from 
which  its  cargo  is  derived  permit  the  economic  use  of  only  a  500- 
ton  freight  train.  The  port  delays  are  too  long.  Such  an  in- 
herent defect  in  harbor  location  can  be  corrected  neither  by  im- 
provement in  loading  facilities,  nor  by  an  increase  in  harbor  depth. 

Density  of  traffic  forces  the  large  freighter  to  large  centers  of 
population  where  factories  exist  which  import  fuel  and  raw  ma- 
terial and  export  manufactured  articles;  and  where  a  wholesale 
trade  is  created  which  stores  products  received  and  retails  them 


ECONOMIES   OF  WATER  TRANSPORTATION          163 

to  smaller  towns;  or  to  localities  where  an  extensive  hinterland 
readily  supplies  an  abundance  of  agricultural  or  mineral  products. 
The  important  ports  of  a  country  are  found  when  a  rich  hinterland 
is  coincident  with  a  large  manufacturing  population.  A  traffic  of 
great  density  then  is  combined  with  a  well-balanced  shipment  of 
exports  and  imports,  and  the  large  freighter  produces  economies 
in  transoceanic  shipments  that  are  impossible  when  operating  to 
minor  ports.  Hence,  there  is  also  an  economic  limitation  to  the 
dimensions  of  vessels  navigating  the  ocean;  but  it  varies  with  the 
traffic  density  of  the  ports  between  which  they  sail.  The  cheapest 
transportation  for  a  minor  port  is  by  a  small  coast  vessel,  which 
transfers  a  cargo  destined  for  export  to  the  larger  vessel  in  the 
first  great  harbor.  The  economies  which  result  from  the  con- 
struction of  the  large  transatlantic  steamships  insure  the  com- 
mercial supremacy  of  a  few  large  harbors.  If  two  harbors  in 
close  proximity  compete  for  transoceanic  trade,  the  law  of  density 
of  traffic  insures  the  survival  of  the  fittest  and  the  relegation 
of  the  unsuccessful  rival  to  the  position  of  a  minor  port.  An 
appropriation  by  the  Federal  Government  for  deepening  a  harbor 
may  aid  the  action  of  natural  laws,  but  it  cannot  overthrow  them. 

The  same  reasoning  applies  not  only  to  the  coast  trade  but  also 
to  the  navigation  of  rivers.  The  large  ocean  steamship  seeks  the 
port  which  has  the  greatest  density  of  traffic,  and  it  leaves  to 
river  and  harbor  craft  the  collection  and  distribution  of  its  cargo 
at  other  towns.  In  river  navigation,  however,  other  principles 
also  are  involved.  The  ocean  vessel  has  to  be  given  sufficient 
strength  to  resist  the  shocks  which  it  sustains  during  storms  on 
the  high  seas.  Wave  action  in  rivers  is  relatively  insignificant. 
A  boat  can  be  constructed  for  river  navigation,  therefore,  much 
cheaper  than  for  ocean  navigation,  so  that  the  annual  interest  on 
the  investment  and  the  cost  of  maintenance  are  much  less  for 
river  craft,  and  this  compensates  in  large  measure  for  the  cost  of 
transferring  the  freight  from  one  class  of  boats  to  the  other. 
The  discharge  and  the  tidal  flow  in  rivers  produce  currents  in 
their  narrow  and  sinuous  channels,  which  are  a  serious  menace 
to  navigation  by  large  vessels.  Insurance  rates  then  become 
prohibitive.  Ocean  vessels,  therefore,  seek  the  city  situated  near 
the  mouth  of  the  river,  and  barge  navigation  connects  this  port 
to  the  interior  towns  on  the  river  bank. 

The  economy  of  mechanical  appliances  for  transferring  a  cargo 


164  RIVERS  AND   HARBORS 

between  boat  and  dock  is  dependent  on  the  character  of  the  freight 
which  is  moved.  Iron  ore,  coal,  grain,  and  other  products  trans- 
ported in  bulk  can  be  handled  much  cheaper  by  devices  like 
Hewlett  Machines  and  Gantry  Cranes,  than  by  manual  labor. 
Package  freight,  when  the  elements  are  of  uniform  size,  also  per- 
mits economic  movement  by  machinery  adapted  to  the  purpose. 
The  vessel  must  be  constructed,  however,  with  numerous  wide 
hatches  to  enable  such  appliances  to  be  utilized  to  their  maximum 
capacity.  With  narrow  hatches  and  with  the  miscellaneous  car- 
goes which  usually  are  carried  on  vessels,  the  ship's  tackle  frequently 
affords  the  quickest  means  of  transferring  merchandise  between 
the  dock  and  boat,  and  the  port  economies  consist  in  measures 
for  rapid  movement  of  freight  to  and  from  the  ship's  side. 

The  wharf  frontage,  the  width  of  wharves,  and  the  water  space 
between  them  are  important  factors  in  port  development.  A 
sufficient  frontage  should  be  afforded  so  that  a  vessel  is  not  de- 
layed unnecessarily  in  obtaining  a  berth  at  which  it  can  load  or 
unload  its  cargo.  In  a  harbor  where  lighterage  is  extensively 
employed,  vessels  can  moor  in  mid-channel  and  discharge  their 
contents  without  access  to  wharves,  and  in  such  harbors  only  a 
limited  wharf  frontage  may  be  necessary.  Thus  at  Manila,  P.  I., 
the  go-downs  which  receive  the  freight  usually  are  located  on  the 
numerous  esteroes  which  intersect  the  city,  and  the  native  casco 
is  a  much  cheaper  method  of  transportation  than  a  caribou  and 
cart  or  the  auto  truck,  so  that  the  deposit  of  a  cargo  on  a  dock 
adds  to  the  cost  of  its  distribution. 

When  there  is  an  extensive  hinterland  which  can  be  reached  by 
river  transportation,  as  at  Hamburg,  Germany,  the  transfer  of 
goods  destined  to  inland  ports  also  can  be  more  economically 
made  direct  from  ship  to  barge,  and  in  such  cases  a  wide  water 
space  between  piers  is  required  so  that  the  barge  can  be  loaded  at 
the  ship  side,  simultaneously  with  the  placing  on  the  pier  of  those 
products  which  are  required  for  local  delivery.  In  New  York 
harbor  there  is  a  large  coast  and  river  traffic  and  an  enormous  local 
distribution  by  lighter. 

The  width  to  be  given  a  wharf  is  primarily  a  function  of  the 
rapidity  with  which  freight  can  be  delivered  to  it  by  the  shipper 
or  removed  from  it  by  the  receiver.  For  example,  on  the  Great 
Lakes,  the  shipping  coal  docks  require  merely  a  width  which  will 
contain  two  railroad  tracks,  one  for  the  loaded  car  whose  contents 


ECONOMIES   OF   WATER  TRANSPORTATION          165 

are  dumped  into  the  vessel,  and  a  return  track  for  the  empties, 
while  the  receiving  dock  has  an  abnormal  width  to  provide  storage 
for  the  coal  received. 

Federal  officials  are  strongly  in  favor  of  wide  docks,  for  the 
purpose  of  storing  goods  for  custom  appraisement,  but  there  re- 
sults an  increased  cost  of  original  construction,  and  of  moving  the 
freight  where  it  is  received  on  piers.  The  narrow  wharves  of 
Hong  Kong  which  have  an  efficient  tramway  system  that  deposits 
the  cargo  promptly  in  neighboring  go-downs  on  land  is  much  more 
economical  in  first  cost  and  in  operation  than  the  large  govern- 
ment pier  at  Manila. 

Where  there  is  not  an  efficient  rail  or  tramway  system  which 
insures  a  prompt  removal  from  the  ship's  side,  sheds  must  be 
erected  on  the  dock  to  receive  both  the  incoming  and  outgoing 
freight  and  to  protect  it  from  the  inclemency  of  the  weather. 
The  width  of  the  shed  becomes  a  function  of  the  dimensions  of 
the  vessels  seeking  the  port  and  varies  with  the  cube  of  their 
length  (4).  Where  the  vessels  moor  against  a  quay  wall  and  the 
sheds  are  erected  on  land,  they  can  readily  be  given  an  ample 
width,  but  when  vessels  moor  at  piers,  the  problem  presents  many 
difficulties.  Thus  at  New  York  harbor  many  of  the  piers  were 
constructed  when  the  draught  of  the  largest  ocean  vessel  did  not 
exceed  26  feet,  and  traffic  becomes  congested  when  vessels  of  a 
draught  of  35  to  40  feet  attempt  to  use  them.  Moreover,  to 
widen  the  piers  so  as  to  obtain  adequate  width  of  shed  would 
diminish  unduly  the  width  of  slip  between  them,  which  is  an 
important  consideration  in  pier  construction,  particularly  at  this 
harbor. 

To  minimize  the  congestion,  some  sheds  are  made  two  stories 
in  height,  the  upper  story  for  outgoing  freight  and  the  lower  for 
incoming  freight.  In  Europe,  the  usual  practice  is  to  utilize  one 
shed  for  the  incoming  freight  and  another  for  the  outgoing  freight. 
When  the  cargo  is  discharged,  the  vessel  is  moved  from  its  berth 
in  front  of  one  to  that  in  front  of  the  other. 

While  facilities  for  moving  freight  by  rail  or  by  truck  can  be  pro- 
vided readily  when  the  shed  is  on  the  mainland,  when  it  is  located 
on  piers  the  question  becomes  a  complicated  one,  since  as  much 
space  as  practical  must  be  devoted  to  piling  the  merchandise  in 
order  to  avoid  congestion.  There  is  a  popular  error  that  con- 
gestion can  be  avoided  by  a  multiplicity  of  rail  tracks  on  a  pier. 


166  RIVERS  AND   HARBORS 

This  is  true  only  when  a  great  mass  of  freight  is  in  transit  to  the 
hinterland,  as  in  handling  coal  and  iron  ore  at  the  docks  of  the 
Great  Lakes.  In  New  York  harbor  a  large  percentage  of  mis- 
cellaneous cargo  requires  local  receipts  and  deliveries,  and  both 
the  truck  and  lighter  are  more  important  factors  than  freight 
cars. 

On  estuaries  the  tidal  fluctuations,  and  on  non-tidal  rivers  the 
height  of  floods,  affect  the  economies  of  loading  and  unloading. 
With  the  large  tidal  fluctuations  in  the  North  Sea,  tidal  basins 
with  their  resultant  delays  in  entering  and  in  departing  are  neces- 
sary. Where  there  is  a  difference  of  elevation  of  over  fifty  feet 
between  flood  and  low  water  stages,  as  at  some  localities  on  the 
Mississippi  River,  a  flexible  means  must  be  devised  for  overcoming 
the  differences  in  elevation  between  the  top  of  the  bank  and  deck 
of  the  vessel.  The  usual  method  adopted  is  a  floating  wharf 
boat  to  receive  the  cargo,  with  a  sloping  bank  over  which  the 
freight  is  hauled  by  animal  traction.  The  substitution  of  ma- 
chinery for  the  horse  or  mule  as  a  means  of  raising  the  freight 
to  the  level  of  the  bank  is  economical  only  when  large  amounts 
are  handled. 


APPENDIX  A 
BIBLIOGRAPHIC  NOTES 

CHAPTER  H 

1.  The  Flow  of  Streams  and  the  Factors  that  Modify  it,  with  Special  Refer- 
ence to  Wisconsin  Conditions,  by  Daniel  Webster  Meade,  C.E.  Bulletin 
of  the  University  of  Wisconsin  No.  425.  Flood  Control  with  Par- 
ticular Reference  to  Conditions  in  the  United  States,  by  Brig.  Gen. 
H.  M.  Chittenden,  retired,  and  a  Discussion.  International  En- 
gineering Congress,  1915,  San  Francisco,  Cal. 

CHAPTER   III 

1.  American  Sewerage  Practice,  by  Metcalf  and  Eddy. 

2.  Merriman's  Civil  Engineers'  Pocketbook,  p.  849. 

3.  The  Currents  of  Lake  Michigan,  Journal  of  the  Western  Society  of  En- 

gineers, Vol.  XXI. 

4.  Extracts  from  Des  Travaux  du  Fleuye  du  Rhin,  by  A.  J.  Ch.  Defontaine, 

Professional  Papers  Corps  of  Engineers,  U.  S.  Army,  and  Department 
at  Large,  Vol.  IX,  p.  528. 

CHAPTER   IV 

1.  Rivieres  a  Courant  Libre,  par  F.  B.  DeMas,  Inspecteur  Ge'ne'ral  des  Fonts 

et  Chaussees,  p.  318. 

2.  The  Flow  of  Sediment  in  the  Mississippi  River  and  its  Influence  on  the 

Slope  and  Discharge;  Professional  Memoirs  Corps  of  Engineers,  U.  S. 
Army,  and  Department  at  Large,  Vol.  VII,  p.  357. 

3.  The   Atchafalaya  —  Some   of   its   Peculiar   Physical   Characteristics  — 

J.  A.  Ockerson  —  Transactions  of  the  American  Society  of  Civil  En- 
gineers, Vol.  XL,  p.  215. 

4.  Regulations  of  Rivers  at  Low  Water,  by  M.  Girardon,  Vlth  International 

Navigation  Congress,  The  Hague,  1894. 

CHAPTER  V 

1.  Considerations  Th6oretiques  sur  les  Jaugeages  des  Cours  d'Eau  a  Fond 

Mobile,  par  M.  R.  Tarvernier,  Inge"nieur  en  Chef  des  Fonts  et  Chausse'es, 
Annales  des  Fonts  et  Chaussees,  1907. 

2.  Report  on  the  Mississippi  River,  by  Humphreys  &  Abbot,  p.  370. 

3.  Engebrisse  der  Untersuchung  der  Hochwasserverhaltniss  im  Deutschen 

Rhein  Gebeit. 

4.  The  Method  of  Finding  Rules  for  Predicting  Floods  in  Water  Courses, 

by  M.  Babinet,  Bulletin  11,  U.  S.  Weather  Bureau. 

5.  Essai  sur  le  Problem  de  1' Annonce  des  Crues  pour  les  Rivieres  des  De'parte- 

ments  de  1'Ardeche,  du  Gard,  et  de  THerault,  par  G.  Lemvine,  An- 
nales des  Fonts  et  Chaussees,  1896. 

6.  Zeitschrift  fur  Bauwesen  bei  Harlacher  und  Richter. 

167 


168  RIVERS   AND   HARBORS 

id  et  du  M 

inieur  des  Fonts  et  Chaussees,  An- 


7.   Etude  Hydrologique  du  Rhin  Allemand  et  du  Main,  des  Crues,  et  leur 
Prevision,  par  M.  Ed.  Maillet,  Inge"nieur 


nales  des  Fonts  et  Chaussees,  1903. 
8.    The  Method  of  Finding  Rules  for  Predicting  Floods  in  Water  Courses, 
by  M.  Babinet,  Bulletin  No.  11,  U.  S.  Weather  Bureau. 


CHAPTER   VI 

1.  Rivieres  a  Courant  Libre,  par  F.  B.  De  Mas,  p.  318. 

2.  Improvement  of  Rivers,  by  Major  William  W.  Harts,  Corps  of  Engineers, 

U.  S.  Army,  Proceedings  of  Xllth  International  Navigation  Congress, 
Philadelphia,  1912. 

3.  Report  of  Chief  of  Engineers,  U.  S.  Army,  1919,  p.  1308. 

4.  The  Regulation  of  Rivers,  by  J.  H.  Van  Ornum,  p.  148. 

5.  Les  Travaux  d' Amelioration  du  Rhone,  par  M.  Armand,  Inge"nieur  en 

Chef  des  Fonts  et  Chaussees,  Annales  des  Fonts  et  Chaussees,  1911, 
p.  544. 

6.  Atti  del  Comitato  Technico  Esecutivo,  Commissione  per  la  Navigazione 

Interna,  Decreto  14  Ottobre,  1903. 

7.  Die  Arbeiten  der  Rheinstrom-Bauverwaltung,  1851- 1900,  by  R.  Jasmund. 

8.  Les  Communications  des  Rhin,  par  M.  Coblenz,  Annales  des  Fonts  et 

Chaussees,  1903. 

CHAPTER  VII 

1.  The  Improvement  of  the  Upper  Mississippi  River;  Journal  of  the  Western 

Society  of  Engineers,  Vol.  XIV,  p.  26. 

2.  Der  Rhein  von  Strasburg  bis  zur  Hollandischen  Grenze,  by  E.  Beyerhause. 
The  Improvement  and  Navigation  of  the  Rhine,  by  J.  W.  Skelly,  Journal 

of  the  Engineers'  Club  of  St.  Louis,  Vol.  V,  No.  4. 

3.  Improvement  of  Navigable  Non-Tidal  Rivers,  by  Prof.  J.  Schlichting, 

translated  by  1st  Lieut.  Fred  V.  Abbot,  Corps  of  Engineers,  U.  S. 
Army,  Engineers'  School  of  Application  Paper,  No.  IX. 

4.  Rivieres  a  Courant  Libre,  par  F.  B.  De  Mas,  p.  171. 

5.  Extracts  from  des  Travaux  du  Fleuve  du  Rhin,  by  A.  J.  Ch.  Defontaine, 

Professional  Memoirs,  Corps  of  Engineers,  U.  S.  Army,  and  Depart- 
ment at  Large,  Vol.  IX. 

6.  A  Resume  of  the  Operations  in  the  First  and  Second  Districts  Improving 

the  Mississippi  River,  by  E.  Eveleth  Winslow,  Major  Corps  of  Engi- 
neers, U.  S.  A.  Occasional  Papers  No.  41,  Engineers'  School,  U.  S.  A. 

7.  Report  of  Chief  of  Engineers,  U.  S.  A.,  1894,  p.  1587,  and  1901,  p.  2243. 

8.  Technical  Methods  of  River  Improvement  as  Developed  on  the  Lower 

Missouri  River,  by  the  General  Government  from  1876  to  1903,  S. 
Waters  Fox;  Transactions  of  the  American  Society  of  Civil  Engineers, 
Vol.  LIV,  p.  280. 

9.  Use  of  Plank  or  Lumber  Apron  Mat  for  Shore  Protection  on  the  Upper 

Mississippi  River  between  the  Wisconsin  River  and  LeClaire,  Iowa, 
by  S.  Edwards,  and  R.  lakisch,  Professional  Memoirs  Corps  of  En- 
gineers, U.  S.  Army,  and  Department  at  Large,  Vol.  VIII,  p.  383. 

10.  Results  of  Experiments  Looking  to  the  Development  of  a  Form  of  Sub- 

aqueous Concrete  Revetment  for  Protection  of  River  Banks  against 
Scour  or  Erosion,  by  Major  E.  M.  Markham,  Corps  of  Engineers, 
U.  S.  A.  Professional  Memoirs  Corps  of  Engineers,  U.  S.  Army,  and 
Department  at  Large,  Vol.  VIII,  p.  731. 

11.  Concrete  Paved  Bank  Revetment,  Missouri  River  Improvement,  by  J.  C. 

Hayden,  Professional  Memoirs  Corps  of  Engineers,  U.  S.  Army,  and 
Department  at  Large,  Vol.  X,  p.  157. 

12.  Commissione  per  la  Navigazione  Interna  Decreto,  14  Ottobre,  1903. 


APPENDIX   A  —  BIBLIOGRAPHY  169 


CHAPTER  VIII 

1.  Report  of  Board  of  Engineers  upon  Applicability  of  Hydraulic  Gates  and 

Dams  on  the  Ohio  River. 

2.  Report  on  Emergency  Dams  of  Swing  Bridge  Type  for  the  Panama  Canal 

at  Gatun  and  Pedro  Maguel,  by  F.  B.  Monniche,  Engineering  News, 
1901,  II,  p.  96. 

3.  Improvement  of  Rivers,  by  Thomas  &  Watts. 

4.  Rolling  Dams,  by  R.  E.  Hilgard,  Transactions  of  the  American  Society 

of  Civil  Engineers,  Vol.  LIV,  Part  D,  p.  439. 

5.  Measuring  Upward  Pressure  Under  a  Dam.     Engineering  News  Record, 

Vol.  84,  p.  1014. 

6.  Water  Power  Development  of  Mississippi  River  Power  Co.  at  Keokuk, 

Iowa,  by  Hugh  L.  Cooper,  Journal  of  the  Western  Society  of  Engineers, 
1912,  p.  213. 

7.  American  Engineers'  Pocketbook,  p.  1045. 

8.  Relation  of  the  Intake  to  Pure  Water  from  the  Great  Lakes,  by  Charles 

B.  Burdick,  M.W.S.E.,  Illinois  Water  Supply  Association,  1911. 

9.  River  and  Canal  Engineering,  by  E.  S.  Bellasis,  p.  54. 

10.  Investigations  of  the  Methods  Best  Suited  for  Surmounting  Great  Dif- 

ferences of  Level  between  the  Reaches  of  Canals,  Xth  International 
Navigation  Congress,  Milan,  1905. 

11.  The  Distribution  of  Stresses  in  Mitering  Lock  Gates,  with  special  refer- 

ence to  the  Gates  of  the  Panama  Canal,  by  Henry  Goldmark,  Tran- 
sactions of  the  American  Society  of  Civil  Engineers,  1917. 
Large  Modern  Lock  Gates,  by  Malcolm  Elliott,  Journal  of  the  Western 
Society  of  Engineers,  Vol.  XXI,  p.  601. 

12.  Service  and  Guard  Gates  for  the  Third  Lock  at  St.  Mary's  Falls  Canal  — 

their  Design,  Construction  and  Cost  —  by  Mr.  Isaac  De  Young,  Pro- 
fessional Memoirs  Corps  of  Engineers,  U.  S.  Army,  and  Department 
at  Large,  Vol.  VIII,  p.  553. 

13.  The  Panama  Canal,  by  Major  G.  W.  Goethals  and  others.     International 

Engineering  Congress,  San  Francisco,  Cal.,  1915. 

14.  Filling  and  Emptying  the  Third  Lock  at  St.  Mary's  Falls  Canal,  by  L.  E. 

Sabin,  Professional  Memoirs  Corps  of  Engineers,  U.  S.  Army,  and 
Department  at  Large,  Vol.  IX,  p.  115. 

15.  Statistical  Report  of  Lake  Commerce  at  St.  Mary's  Falls  Canal. 


CHAPTER   IX 

1.  Reports  of  Chief  of  Engineers,  U.  S.  Army. 

2.  Improvement  of  the  Livingston  Channel,  Detroit  River,  by  C.  Y.  Dixon, 

Professional  Memoirs  Corps  of  Engineers,  U.  S.  Army,  and  Depart- 
ment at  Large,  Vol.  VIII,  p.  760. 

3.  Improvement  of  the  Upper  Mississippi  River,  Journal  of  The  Western 

Society  of  Engineers,  Vol.  XIV,  p.  26. 

4.  Excavation  Machinery,  Methods  and  Cost,  by  McDaniels.    Prelim,  Dredges 

and  Dredging. 

5.  Dredges  and  Dredging  on  the  Mississippi  River,  by  J.  A.  Ockerson,  Transac- 

tions of  the  American  Society  of  Civil  Engineers,  Vol.  XL,  p.  215. 

6.  Chicago  Drainage  Canal,  by  Isham  Randolph,  Chief  Engineer,  Sanitary 

District  of  Chicago,  Journal  of  the  Western  Society  of  Engineers,  Vol. 
II,  page  657. 

7.  Der  Arbeiten  der  Rheinstrom-Bauverwaltung,  1851-1900,  by  R.  Jasmund. 

8.  Guthman  on  the  Iron  Gates  of  the  Danube,  Minutes  of  Proceedings  of 

the  Institution  of  Civil  Engineers,  Vol.  CXIX. 

9.  Project  for  Improvement  of  Hell  Gate,  by  Col.  Newton  —  Report  of 

Chief  of  Engineers,  U.  S.  Army,  1868,  p.  738. 


170  RIVERS   AND    HARBORS 

10.  Project  for  Removal  of  Blossom  Rock,  by  Lieut.  Col.  Alexander.     Report 

of  Chief  of  Engineers,  U.  S.  Army,  1871,  p.  929. 

11.  Recent  Lower  Mississippi  Valley  Waterway  Improvements,  by  Major 

Clark  S.  Smith,  Professional  Memoirs  Corps  of  Engineers,  U.  S.  Army, 
and  Department  at  Large,  Vol.  IV,  p.  367. 

The  Flow  of  Sediment  in  the  Mississippi  River  and  its  Influence  on  the 
Slope  and  Discharge,  same  Vol.  VII,  p.  374. 

12.  Coast  Lighting  in  the  United  States,  by  D.  W.  Lockwood.     Transactions 

of  the  American  Society  of  Civil  Engineers,  Vol.  LIV,  Part  B,  p.  43. 

13.  Memoir  upon  the  Illumination  and  Beaconage  of  the  Coast  of  France,  by 

M.  Leonce  Reynaud,  translated  by  Major  Peter  C.  Hains,  Corps  of 
Engineers,  U.  S.  Army. 


CHAPTER   X 

1.  Forests  and  Reservoirs  in  their  Relation  to  Stream  Flow,  by  H.  M. 

Chittenden.     Transactions  of  the  American  Society  of  Civil  Engineers, 
Vol.  LXII,  p.  145. 

2.  Report  of  Board  of  Engineers  upon  Matters  Connected  with  the  Opera- 

tions of  Reservoirs  at  the  Headwaters  of  the  Upper  Mississippi  River; 
Report  of  the  Chief  of  Engineers,  U.  S.  Army,  1906,  p.  1442. 

3.  Report  of  Chief  of  Engineers,  U.  S.  Army,  1915,  p.  1888. 

4.  The  Levee  Theory  of  the  Mississippi  River,  by  D.  M.  Harrod  and  others; 

Transactions  of  the  American  Society  of  Civil  Engineers,  Vol.  LI,  p. 
331. 

5.  Report  of  Mississippi  River  Commission,  1896,  p.  3573  and  1899,  p.  3373. 

6.  Le  Danube  dans  la  Basse  Autriche,  par  M.  Armand,  Ingenieur  en  Chef  des 

Ponts  et  Chaussees,  Annales  des  Ponts  et  Chaussees,  1910. 


CHAPTER   XI 

1.  The  Control  of  Hydraulic  Mining  in  California  by  the  Federal  Govern- 

ment, William  W.  Harts,  American  Society  of  Civil  Engineers. 
Vol.  LVI1,  p.  1. 

The  Failure  of  the  Yuba  River  Debris  Barrier  and  the  Efforts  Made  for 
its  Maintenance,  H.  H.  Wadsworth,  same.     Vol.  LXXI,  p.  217. 

2.  Commissione  per  la  Navigazione  Interna,  Decreto  14  Ottobre,  1903. 

3.  Decrease  of  Water  in  Springs,  Creeks  and  Rivers  Contemporaneously 

with  the  Increase  in  the  Heights  of  Floods  in  Cultivated  Countries,  by 
Sir  Gustav  Wex,  translated  by  G.  Weitzel,  Major  of  Engineers,  Brevet 
Major  General,  U.  S.  Army. 

4.  Influence  of  Deforestation  and  the  Drying  Up  of  Marshes  on  the  Sphere 

of  Influence  and  on  the  Performance  of  Rivers,  Permanent  International 
Association  of  Navigation  Congresses,  Xth  Congress,  Milan,  1905. 

5.  II  Po  nelle  Effemeridi  di  un  Secolo,  Ing.  G.  Fantoli,  Atti  Delia  Societa 

Italiana  per  il  Progresso  delle  Scienze,  VI  Rimione,  Genova,  Ottobre, 
1912. 

6.  The  Rise  of  the  Bed  of  the  Yellow  River  by  the  Deposit  of  Sediment,  by 

Brig.  Gen.  William  L.  Sibert,  U.  S.  Army.  Professional  Memoirs  Corps 
of  Engineers,  U.  S.  Army,  and  Department  at  Large,  Vol.  VII,  p.  359. 

7.  Notes  on  the  River  Nile,  A.  W.  Robinson,  Canadian  Society  of  Civil 

Engineers,  February,  1912. 

8.  Forests,  Reservoirs  and  Stream  Flow,  Gifford  Pinchot.     Transactions  of 

the  American  Society  of  Civil  Engineers,  Vol.  LXII,  p.  456. 
Pros  and  Cons  on  the  Forest  and  Flood  Question,  by  Thomas  B.  Roberts, 
Professional  Memoirs  Corps  of  Engineers,  U.  S.  Army,  and  Department 
at  Large,  Vol.  V,  p.  568. 


APPENDIX  A  —  BIBLIOGRAPHY  171 

9.  Forests  and  Reservoirs  in  their  Relation  to  Stream  Flow.  H.  M.  Chit- 
tenden.  Transactions  of  the  American  Society  of  Civil  Engineers, 
Vol.  XL,  p.  215. 

The  Control  of  River  Floods.     Professional  Memoirs  Corps  of  Engineers, 
U.  S.  Army,  and  Department  at  Large,  Vol.  V,  p.  414. 

10.  Report  of  the  Miami  Conservation  Commission. 

11.  How  the  Ancients  would  have  Controlled  the  Mississippi  and  its  Tribu- 

taries, by  Sir  William  Wilcocks,  and  Discussion.     Proceedings  of  the 
Engineers  Society  of  Western  Pennsylvania,  Vol.  30. 

12.  The  Use  of  Outlets  for  Reducing  Flood  Heights,  by  Mr.  J.  A.  Ocherson, 

Member  Mississippi  River  Commission.  Professional  Memoirs  Corps 
of  Engineers,  U.  S.  Army,  and  Department  at  Large,  Vol.  VII,  p.  603. 
A  Compilation  of  Experiences  and  Conclusions  of  Eminent  Authorities 
as  to  the  Utility  of  Outlets  in  Reducing  the  Height  of  Floods.  Citing 
all  Opinions  found  for  or  against.  Same,  p.  719. 

13.  Flood  Control  Sacramento  and  San  Joaquin  River  Systems,  California. 

House  of  Representatives  Doc.  No.  81,  62d  Congress,  1st  Session  and 
Doc.  No.  5,  63d  Congress,  1st  Session. 

14.  Separation   of  Red  and  Atchafalaya  Rivers   from   Mississippi   River. 

House  of  Representatives  Doc.  No.  841,  63d  Congress,  2d  Session. 

15.  Standard  Levee  Sections,  H.  St.  L.  Coppe"e.     Transactions  of  the  Ameri- 

can Society  of  Civil  Engineers,  XXXIX,  p.  191. 

16.  Levee  Building  Machine,  by  Major  J.  R.  Slattery,  Corps  of  Engineers, 

U.  S.  Army.     Professional  Memoirs  Corps  of  Engineers,  U.  S.  Army, 
and  Department  at  Large,  Vol.  VIII,  p.  320. 

17.  Report  of  the  Miami  Conservation  Commission. 


CHAPTER   XII 

1.  Practical  Manu-v  of  Tides  and  Waves,  by  W.  H.  Wheeler,  p.  94. 

2.  Same  —  p.  97. 

3.  Same  —  p.  94. 

4.  Same  —  p.  143. 

5.  Same  —  p.  91. 

6.  Rivers  and  Canals,  by  Liveson  Francis  Vernon-Harcourt,  p.  232. 

7.  History  of  the  Conversion  of  the  River  Clyde  into  a  Navigable  Water- 

way, by  James  Dean.     Transactions  of  the  American  Society  of  Civil 
Engineers,  Vol.  XXIX,  p.  128. 

8.  Rivers  and  Canals,  by  L.  F.  Vernon-Harcourt,  p.  235. 


CHAPTER   XIII 

1.  Principles  and  Practices  of  Harbour  Construction,  by  William  Shields, 

F.R.S.,  p.  52. 

2.  Practical  Manual  of  Tides  and  Waves,  by  W.  H.  Wheeler,  p.  126. 

3.  Rivers  and  Canals,  by  Liveson  Francis  Vernon-Harcourt,  p.  307. 

4.  History  of  the  Jetties  at  the  Mouth  of  the  Mississippi  River,  by  E.  L. 

Corthell. 

5.  The  Passes  of  the  Mississippi  River,  by  Lieut.  Col.  Edward  H.  Schultz, 

Corps  of  Engineers,  U.  S.  Army.     Professional  Memoirs  Corps  of  En- 
gineers, U.  S.  Army,  and  Department  at  Large,  Vol.  IX,  p.  135. 

6.  Currents  at  and  near  Mouth  South  West  Pass  Mississippi  River,  by  Mr. 

T.  E.  Lipsey,  Ass't  Engineer  in  Charge,  Professional  Memoirs  Corps 
of  Engineers,  U.  S.  Army,  and  Department  at  Large,  Vol.  XI,  p.  65. 

7.  Rivers  and  Canals,  by  L.  F.  Vernon-Harcourt,  p.  305. 

8.  Report  of  Chief  of  Engineers,  U.  S.  Army,  1919,  p.  296. 

9.  Harbours  and  Docks,  by  L.  F.  Vernon-Harcourt,  p.  68. 
10.   Same  —  p.  157. 


172  RIVERS  AND   HARBORS 

11.  Seacoast  Harbors  of  the  United  States,  Cassius  E.  Gillette.     Transactions 

of  the  American  Society  of  Civil  Engineers,  Vol.  LIV,  Part  A,  p.  297. 

12.  Report  of  Chief  of  Engineers,  U.  S.  Army,  1919,  p.  672. 

13.  The  Reaction  Breakwater  as  Applied  to  the  Improvement  of  Ocean  Bars, 

Louis  M.  Haupt,  Transactions  of  the  American  Society  of  Civil  En- 
gineers, Vol.  XLII,  p.  45. 

14.  Improvement  of  the  Mouth  of  the  Columbia  River,  by  Mr.  Gerald  Bag- 

nell.  Assistant  Engineer,  Professional  Memoirs  Corps  of  Engineers, 
U.  S.  Army,  and  Department  at  Large,  Vol.  VIII,  p.  687. 

CHAPTER  XIV 

1.  Report  of  Royal  Commission  on  Harbours  of  Refuge,  Vol.  I,  p.  240. 
The  Design  and  Construction  of  Harbours,  by  Thomas  Stevenson.  F.  R. 

S.  E.,  1864,  p.  121. 

2.  House  of  Representatives  Doc.  No.  62,  59th  Congress,  1st  Session. 

3.  The  Breakwater  at  Buffalo,  N.  Y.,  Emile  Low.     Transactions  of  the 

American  Society  of  Civil  Engineers,  Vol.  LII,  p.  73. 

4.  Harbors  on  Lake  Erie  and  Ontario,  Dan  C.  Kingman.     Transactions  of 

the  American  Society  of  Civil  Engineers,  Vol.  LIV,  p.  237. 

5.  Report  of  Chief  of  Engineers,  U.  S.  Army,  1879,  p.  1562. 

6.  Harbor  Construction  at  the  Port  of  Manila,  P.  I.     Occasional  Paper  No. 

21,  Engineers'  School,  U.  S.  Army. 

7.  Harbours  and  Docks,  by  L.  F.  Vernon-Harcourt. 

8.  The  Delaware,  Sandy  Bay  and  San  Pedro  Breakwaters,  C.  H.  McKinstry. 

Transactions  of  the  American  Society  of  Civil  Engineers,  Vol.  LIV, 
Part  A,  p.  325. 

9.  Concrete  Blocks  at  Osaka  Harbour  W°rk>  Japan,  S.  Shima.     Transac- 

tions of  the  American  Society  of  Civil  Engineers,  Vol.  LIV,  Part  A, 
p.  237. 

10.  Principles  and  Practices  of  Harbour  Construction,  by  William  Shields, 

F.R.S.,  p.  178. 

11.  Same  — p.  205. 

12.  Breakwater  at  Sandy  Bay,  Cape  Ann,  Mass.,  by  Col.  E.  Creighill,  Corps 

of  Engineers,  U.  S.  Army,  Professional  Memoirs  Corps  of  Engineers, 
U.  S.  Army,  and  Department  at  Large,  Vol.  VIII,  p.  587. 

13.  Cost  of  Crib  Construction,  Brief  Methods  of  Preparing  Estimate,  by 

J.  A.  M.  Liljencrantz,  Journal  of  Western  Society  of  Engineers,  VoL 
IV,  p.  361. 

14.  Concrete  Steel  Caissons,  their  Development  and  Use  for  Breakwaters, 

Piers  and  Revetments,  by  W.  V.  Judson,  Journal  of  the  Western 
Society  of  Engineers,  1909. 

CHAPTER  XV 

1.  Freight  Train  Resistance  —  Its  Relation  to  Car  Weight,  Bulletin  43, 

University  of  Illinois. 

2.  Influence  de  la  Capacite"  des  Bateaux  sur  les  Frais  de  Transports  par 

Canaux  par  M.  Galliot,  Inspecteur  General  des  Ponts  et  Chaussees, 
Annales  des  Ponts  et  Chaussees,  1920. 

3.  Utilization  of  the  Navigation  of  Large  but  Shallow  Rivers.     Xllth  Inter- 

national Congress  of  Navigation,  No.  48,  Philadelphia,  1912. 

4.  Wharf  Equipment,  by  Ray  S.  MacElwee,  Ph.D.,  Professional  Memoirs 

Corps  of  Engineers,  U.  S.  Army,  and  Department  at  Large,  Vol.  X, 
p.  820. 


APPENDIX  B 
EXAMPLES   OF  FLOOD  PREDICTION 

Cairo  at  Junction  of  Ohio  and  Mississippi  Rivers 

The  problem  can  be  stated  as  follows :  When  the  crest  of  a  flood 
passes  Cincinnati  on  the  Ohio  River,  what  height  will  be  recorded 
on  the  gage  at  Cairo,  at  the  mouth  of  the  Ohio,  six  days  thereafter? 
The  data  available  for  its  solution  are  the  gage  readings  at 
Cincinnati,  498  miles  above  Cairo  on  the  Ohio  River,  at  Chatta- 
nooga, 509  miles  above  Cairo  on  the  Tennessee,  at  Nashville, 
246  miles  above  Cairo  on  the  Cumberland,  and  at  St.  Louis,  191 
miles  above  Cairo  on  the  Mississippi,  on  the  date  the  crest  of  the 
flood  passes  Cincinnati.  Gage  records  at  these  places  are  avail- 
able for  a  period  of  over  forty  years. 

If  there  are  plotted  the  gage  heights  of  the  flood  at  Cairo  six 
days  after  the  flood-wave  passes  Cincinnati,  using  the  height  at 
Cincinnati  as  abscissae,  a  curve  can  be  drawn  giving  the  mean 
gage  relation  between  the  two  localities,  as  shown  hi  Fig.  4. 
This  curve  shows  merely  that  the  mean  of  all  the  floods  that  have 
passed  Cincinnati  at  a  given  height  has  attained  a  height  at  Cairo 
as  shown  by  the  curve.  Thus  a  flood  of  fifty  feet  at  Cincinnati 
will  produce  a  mean  flood  height  at  Cairo  of  41.9  feet.  But  in 
order  that  the  flood  at  Cairo  shall  actually  attain  that  height, 
average  conditions  must  obtain,  not  only  in  the  slope  of  the  Ohio 
River,  but  in  the  discharge  of  the  tributaries. 

Similarly,  by  plotting  the  heights  which  the  Cairo  gage  recorded 
the  day  the  crest  of  the  flood  passed  Cincinnati,  a  curve  is  obtained 
which  indicates  the  mean  difference  in  gage  heights  between  Cairo 
and  Cincinnati  on  that  date.  On  the  Ohio  River  this  curve  is 
parallel  to  the  curve  of  mean  gage  relations  and  about  5  feet  be- 
low it,  i.e.,  if  on  the  day  the  crest  of  the  flood  passes  Cincinnati, 
the  Cairo  gage  reads  5  feet  less  than  that  shown  by  the  curve  of 
mean  gage  relations,  the  river  slope  is  normal  and  the  crest  of  the 
flood  at  Cairo  six  days  thereafter  will  conform  to  that  shown  by 
the  curve  if  there  are  no  perturbations  from  the  tributaries.  If, 

173 


174 


RIVERS  AND   HARBORS 


FIG.  4 


APPENDIX   B  —  FLOOD   PREDICTION  175 

however,  on  the  day  the  crest  of  the  flood  passes  Cincinnati,  the 
reading  of  the  Cairo  gage  is  higher  or  lower  than  5  feet  below  that 
indicated  by  the  curve  of  mean  gage  relations,  the  crest  of  the 
flood  will  exceed  or  be  less  than  the  height  shown  by  the  curve 
and  by  an  amount  in  this  particular  case  found  to  be  equal  to  one- 
half  the  difference. 

The  tributaries  will  also  have  a  disturbing  influence  unless  they 
also  conform  to  average  conditions,  and  similar  curves  have  been 
constructed  also  for  each  tributary  giving  the  average  height  it 
has  attained  when  the  crest  of  the  flood  passed  Cincinnati.  If  the 
actual  heights  differ  from  the  mean,  corrections  have  to  be  ap- 
plied, which  for  the  upper  Mississippi  River  at  St.  Louis  are  found 
to  be  db  1/6  the  difference,  for  the  Tennessee  River  at  Chatta- 
nooga ±1/15,  and  for  the  Cumberland  River  at  Nashville  ±  1/20. 

There  remains,  however,  a  large  area  of  country  (over  50,000 
square  miles)  drained  by  the  numerous  rivers  emptying  into  the 
Ohio  between  Cincinnati  and  the  mouths  of  the  Cumberland  and 
Tennessee  rivers  which  affects  the  computations.  The  Wabash 
is  the  largest  of  these  rivers.  Employing  its  flow  as  an  indicator 
for  the  others  as  M.  Belgrand  utilized  certain  tributaries  of  the 
Seine  in  his  flood  predictions  for  Paris,  it  is  found  that  when  the 
Wabash  River  at  Mt.  Carmel,  221  miles  from  Cairo,  is  rising  or 
falling  more  than  six  inches  a  day,  the  height  of  the  Cairo  flood 
is  increased  or  diminished  about  one  foot.  The  data  for  the 
Wabash  River  have  been  published  by  the  Mississippi  River  Com- 
mission during  the  past  twenty  years  only,  so  that  the  correction 
can  be  but  partially  applied  in  the  tables. 

As  it  requires  only  from  two  to  three  days  for  the  flood-wave  to 
be  transmitted  from  St.  Louis  and  Nashville  to  Cairo,  and  about 
five  days  for  it  to  be  transmitted  from  Chattanooga,  the  readings 
of  the  gages  on  the  day  the  crest  of  the  flood  passes  Cincinnati 
are  not  those  which  produce  the  crest  of  the  flood  at  Cairo. 
They  should  be  increased  or  diminished  by  the  rise  or  fall  during 
the  interval  which  must  elapse  before  their  waters  will  be  in  con- 
junction with  the  crest  of  the  Ohio  River  flood.  To  obtain  ac- 
curate results  at  Cairo  a  flood  prediction  therefore  is  required  at 
these  localities,  and  the  rise  or  fall  during  the  preceding  day 
for  this  reason  is  added  to  or  subtracted  from  their  readings. 
This  ordinarily  gives,  within  a  few  feet,  the  height  of  the  wave  in 
the  river  which  affects  the  flood  crest  at  Cairo,  but  occasionally 


176  RIVERS  AND   HARBORS 

an  error  of  from  5  to  10  feet  in  the  gage  readings  occurs  from 
the  resulting  faulty  predictions,  causing  an  error  from  0.5  foot  to 
one  foot  in  the  flood  height  at  Cairo,  as  is  shown  in  the  last  column 
on  Table  1. 

There  is,  however,  an  exception  to  the  rule  which  must  be  care- 
fully guarded  against.  If  the  river  is  falling  at  Cairo,  while  it  is 
rising  at  Cincinnati,  it  usually  indicates  that  a  flood  from  some  of 
the  other  rivers  is  passing  Cairo,  on  which  that  of  the  Ohio  River 
is  merely  a  perturbation.  If  the  flood  originates  in  the  upper 
Mississippi  River,  which  is  normally  the  case,  its  height  at  Cairo 
can  be  determined  by  a  similar  set  of  curves  shown  in  Fig.  4 
with  St.  Louis  the  controlling  gage.  The  prediction  can  be  made 
only  three  days  in  advance,  and  the  heights  of  the  gages  at 
Cincinnati  and  Chattanooga  used  are  those  recorded  three  days 
prior  to  the  crest  of  the  flood  at  St.  Louis.  Theoretically  the 
readings  of  the  gages  at  Evansville  on  the  Ohio  River,  179  miles 
from  Cairo,  and  at  Johnsonville  on  the  Tennessee,  141  miles  from 
Cairo,  on  the  day  the  flood  passes  St.  Louis,  are  preferable  to  the 
ones  employed  on  those  rivers,  but  the  records  of  these  gages  were 
not  available  when  the  computations  were  first  made  and  have 
been  published  only  recently  in  the  daily  bulletins  of  the  Weather 
Bureau. 

The  gage  at  Cairo  also  may  fall  while  that  at  Cincinnati  is 
rising,  due  to  an  ice  gorge  in  the  Ohio  River,  as  in  January,  1918. 
Such  irregularities  are  incapable  of  computation. 

If  the  main  flood-wave  passing  Cincinnati  begins  to  fall,  and 
within  two  or  three  days  a  perturbation  from  one  of  the  upper 
tributaries  causes  a  second  slight  rise,  the  computations  should  be 
based  on  the  main  rise.  Before  the  flood  reaches  Cairo  the  per- 
turbation will  be  absorbed  in  the  general  flood  and  merely  prolong 
its  duration. 

St.  Louis  at  the  Junction  of  the  Mississippi  and  Missouri  Rivers 

A  similar  set  of  curves  has  been  deduced  (shown  in  Fig.  5)  for 
determining  the  height  of  a  flood  at  St.  Louis  three  days  after  its 
crest  passes  Kansas  City,  388  miles  above  the  mouth  of  the 
Missouri  River,  with  perturbations  of  the  upper  Mississippi  com- 
puted from  the  gage  at  Hannibal,  and  of  the  Illinois  River  from 
that  at  Peoria. 


APPENDIX   B  —  FLOOD   PREDICTION  177 


FIG.  5 


178  RIVERS  AND   HARBORS 

There  are,  however,  three  important  tributaries  of  the  Missouri 
River,  between  Kansas  City  and  its  mouth,  on  which  regular  gage 
stations  should  be  established  and  maintained  for  a  number  of 
years,  to  enable  accurate  predictions  of  the  floods  at  St.  Louis  to 
be  made;  i.e.,  the  Grande  River,  draining  7185  square  miles, 
the  Osage,  draining  15,375  square  miles,  and  the  Gasconade,  3553 
square  miles.  The  floods  in  these  rivers  are  sometimes  very  vio- 
lent, the  rivers  rising  twenty  feet  in  a  day,  and  with  a  moderate  flood 
in  the  Missouri  River,  their  effect  singly  or  combined  may  be  large. 
There  is  no  satisfactory  indicator  of  their  combined  action. 
Hermann  on  the  Missouri  River,  103  miles  from  its  mouth,  gives  an 
invariable  warning  that  there  is  an  abnormal  flow  in  the  Missouri 
River,  by  a  rise  of  over  two  feet  on  the  day  the  crest  of  the  flood 
passes  Kansas  City,  and  it  also  measures  the  extent  of  the  rise, 
but  on  the  same  day  the  flood  passes  St.  Louis  and  therefore  too 
late  to  be  of  value  in  predicting  floods.  During  the  years  1915, 
'16,  and  '17,  the  Weather  Bureau  published  the  gage  readings  at 
Chillicothe  on  the  Grande  River,  62  miles  from  its  mouth,  and  at 
Arlington  on  the  Gasconade,  108  miles  from  its  mouth.  It  has 
also  occasionally  published  the  readings  at  Bagnelle,  70  miles 
from  the  mouth  of  the  Osage  River.  These  limited  observations 
clearly  indicate  that  if  continuous  records  were  published  for  a 
number  of  years  at  the  same  stations  on  these  rivers,  a  satisfactory 
flood  prediction  could  be  made  for  St.  Louis  for  lower  stages  than 
those  indicated  in  Table  3. 

Mississippi  Floods  at  the  Mouth  of  White  River 

An  application  of  the  same  principles  has  been  made  at  the 
mouth  of  White  River,  as  shown  in  Table  4,  with  Cairo  as  the 
origin  of  floods,  393  miles  above  it,  and  for  the  tributaries  using 
the  gages  at  Little  Rock  on  the  Arkansas  River,  174  miles  from 
its  mouth,  and  Jacksonport  on  the  White  River,  264  miles  from 
its  mouth.  In  this  case  the  time  of  transmission  of  the  flood  is 
variable,  —  it  is  about  5  days  for  floods  in  the  Mississippi  less 
than  40  feet  in  height,  about  10  days  for  those  over  45  feet.  If  a 
crevasse  occurs  in  the  levee  line  in  the  St.  Francis  Basin  it  may 
be  increased  to  two  weeks.  In  the  table,  the  computations  are 
based  on  the  transmission  of  the  flood  from  Cairo  to  the  mouth  of 
White  River  in  5  days,  for  floods  not  exceeding  42  feet  in  height, 


APPENDIX   B  —  FLOOD   PREDICTION  179 

and  in  10  days  for  those  exceeding  46  feet.  Between  42  feet  and 
46  feet,  the  results  for  a  movement  of  the  flood  wave  in  both  5  and 
10  days  are  given.  On  account  of  the  frequency  of  breaks  in 
the  levee  line,  prior  to  1910,  the  computations  are  confined  to  the 
past  ten  years. 


APPENDIX  C 

INFLUENCE  OF  FORESTS  UPON  STREAMS 
Verbal  Note 

Foreign  Office 

No.  II  S  7540 

81331 

Referring  to  the  Verbal  Note  of  the  27th  ult.,  the  Foreign  Office 
begs  to  transmit  to  the  Embassy  of  the  United  States  of  America 
the  enclosed  copy  of  an  expert  opinion  on  the  influence  of  forests 
upon  streams,  rendered  by  the  instructor  in  the  Academy  at 
Eberswalde,  Professor  Dr.  Schubert. 

The  Royal  Prussian  Minister  of  Agriculture,  at  whose  instiga- 
tion the  above-mentioned  opinion  was  given,  has  no  further  ma- 
terial on  hand.  But  more  detailed  information  on  the  subject 
may  be  found  in  a  paper  on  the  drainage  from  forests,  read  by 
Professor  Dr.  Vater  of  Tharandt,  a  separate  impression  of  which, 
from  the  report  of  the  49th  meeting  of  the  Sachsischen  Forst- 
verein  in  Marienberg  i.  S.  1905,  was  published  in  Freiberg  i.  S. 
1905  by  Craz  &  Gerlach  (Sch.  Stettner),  and  in  the  report  by 
Mr.  Hermann  in  the  "Forstliche  Rundschau"  of  1906,  p.  45, 
published  by  S.  Neumann  in  Neudamm. 

Berlin,  November  7th,  1908 
To  the  Embassy  of  the 
United  States  of  America. 

Copy  from  S  7540. 
Meteorological  Section  of  the 
Experimental  Department  of  Forestry. 

Discussions  about  the  influence  of  the  destruction  of  forests  and 
the  drainage  of  swamps  upon  the  course  and  the  water-supply  of 
streams,  for  which  the  representatives  of  the  different  countries 
had  submitted  reports,  took  place  at  the  Congress  on  navigation 
at  Milan  in  1905.  The  views  in  regard  to  the  action  of  forests 

180 


APPENDIX   C  —  FORESTS  181 

were  rather  divided,  and  the  members  finally  contented  themselves 
with  the  following  joint  resolution: 

"The  Congress  expresses  the  wish  that  those  Governments 
which  have  not  done  so  before,  may  now  issue  distinct  and  rig- 
orous instructions  about  the  conservation  of  the  forests  still  ex- 
tant, about  the  protection  of  the  mountain  districts  and  about  the 
afforestation  of  waste  lands,  so  as  to  avoid  the  damage  to  navigable 
water-courses,  resulting  from  the  formation  and  the  movement  of 
alluvial  detritus. " 

Interesting  numerical  results  were  embodied  in  the  report  by 
M.  E.  Lauda,  Director  of  Public  Works  and  Chief  of  the  Hydro- 
graphical  Central  Bureau  in  Vienna,  containing  measurements  as 
to  precipitation  and  drainage  of  the  Bistritzka  and  the  Seniza 
creek-basins.  Both  these  valleys  show  great  similarity  in  regard 
to  area,  shape  of  surface,  and  permeability  of  soil,  the  last  named  of 
which  they  possess  only  in  a  moderate  degree.  There  is  also  no 
marked  difference  in  the  angle  of  their  banks  and  in  their  eleva- 
tion above  sea-level.  The  two  localities  are  about  20  kilometers 
distant  from  one  another.  The  Bistritzka  creek  flows  in  a  di- 
rection from  east  to  west,  the  Seniza  creek  from  south  to  north. 
In  both  valleys  meadows  and  pastures  are  in  preponderance  of 
the  tilled  ground.  But  while  these  conditions  are  fairly  similar, 
the  area  of  forest  in  the  district  of  the  Bistritzka  is  about  1.8 
times  greater  than  that  in  the  district  of  the  Seniza.  The  annual 
precipitation  is  nearly  equal  for  both  valleys,  that  of  the  Bistritzka 
basin  being  a  trifle  heavier  in  summer. 

Highest  elevation  Area  of  district 

above  sea-level        in  square         Area  of  forest,  Drainage, 
(in  meters)          kilometers  per  cent         per  cent 

Bistritzka       .      .       912  63.8  48  28 

Seniza       ...       923  74.0  27  42 

The  drainage,  in  proportion  to  the  precipitation,  from  the  heavier 
timbered  district  is  considerably  smaller,  amounting  to  only  two- 
thirds  of  that  of  the  other  valley. 

In  wide  districts,  during  the  dry  period  of  summer,  the  propor- 
tion of  drainage  was  of  the  same  inconsiderable  quantity  of  about 
5  per  cent. 

An  article  by  F.  Umfahrer,  on  the  report  cited  above  and  on  the 
transactions  of  the  congress  on  navigation,  is  to  be  found  in  the 


182  RIVERS   AND    HARBORS 

"  Oesterreichische  Wochenschrift  fur  den  offentlichen  Baudienst" 
(Austrian  Weekly  Magazine  for  Public  Works),  XII,  1906,  p.  176. 

According  to  the  measurements,  taken  by  the  Prussian  State- 
institution  for  Hydrology  (Prussische  Landesanstalt  fur  Gewas- 
serkunde) ,  it  appears  that  in  a  number  of  river  basins  in  Northern 
Germany  the  proportion  of  drainage  for  the  heavier  timbered 
districts  is  generally  larger  than  that  for  those  poorer  in  forests. 

From  a  comparison  of  several  affluents  of  the  Vistula  the  follow- 
ing values  are  derived: 

Precipitation  Forest,  Drainage, 

in  millimeters         per  cent  per  cent 

Ferse,  Dreweuz     ....       540  17.5  27.0 

Brahe,  Schwarzwasser      .      .       555  30.1  34.1 

There  the  permeability  of  the  ground  plays  an  essential  part, 
being  greater  in  wooded  localities  possessing  a  more  sandy  soil, 
and  naturally  increasing  the  amount  of  drainage. 

The  forests  in  the  lowlands  of  Northern  Germany  alter  the 
drainage  in  a  sense  exactly  opposite  to  that  of  the  results  of  the 
Austrian  experiments  and  so  we  shall  have  to  agree  to  the  opinion 
of  G.  Keller  that  the  influence  of  the  forests  on  the  drainage  pro- 
ceedings is  being  hidden  by  other  causes  more  powerful  in  their 
effects. 

EBERSWALDE,  1908. 
Signed: 

PROF.  DR.  T.  SCHUBERT. 


TABLES 


184 


RIVERS   AND   HARBORS 


++ 


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OHIO   RIVER   FLOODS 


185 


4- 


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186 


RIVERS  AND   HARBORS 


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MISSOURI   RIVER  FLOODS 


187 


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188 


RIVERS  AND   HARBORS 


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MISSISSIPPI    RIVER   FLOODS 


189 


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